Here is how to unlock two secret characters (Darkseid, Shao Kahn)  in Mortal Kombat vs DC Universe:

1. To unlock Darkseid: Finish the DC Universe side of the story mode. Then hold down the RB button (on Xbox 360) / R1 button (on PS3) on the character select screen. (Unfortunately, he has no Heroic Brutalities.)
2. To unlock Shao Kahn: Finish the Mortal Kombat side of the story mode. Then hold down the RB button (on Xbox 360) / R1 button (on PS3) on the character select screen. (Unfortunately, he has no Fatalities.)

DC Chapter 1 done (10 points): Complete Story Mode DC Chapter 1.
DC Chapter 3 done (15 points): Complete Story Mode DC Chapter 3.
DC Chapter 5 done (20 points): Complete Story Mode DC Chapter 5.
MK Chapter 2 done (10 points): Complete Story Mode MK Chapter 2.
MK Chapter 4 done (15 points): Complete Story Mode MK Chapter 4.
MK Chapter 6 done (20 points): Complete Story Mode MK Chapter 6.
Story mode MK (50 points): Complete the MK side of Story Mode.
Story mode DC (50 points): Complete the DC side of Story Mode.
DC Arcade Champion (50 points): Complete Arcade Mode on the MK side.
MK Arcade Champion (50 points): Complete Arcade Mode on the DC side.
Dark Kahn! (50 points): Unlock Both Characters.
Compeditor (50 points): Play 200 Versus Matches.
Fatality! (10 points): Do anyone’s fatality.
Brutality! (10 points): Brutality!
The Finisher (100 points): Perform all finishing moves in the game.
Combo Champion (20 points): Perform a 10 hit combo in arcade mode.
Baraka Expert (5 points): Complete Baraka’s expert combos.
Batman Expert (5 points): Complete Batman’s expert combos.
Catwoman Expert (5 points): Complete Catwoman’s expert combos.
Deathstroke Expert (5 points): Deathstroke Expert.
Flash Expert (5 points): Complete Flash’s expert combos.
Greenlantern Expert (5 points): Complete Greenlantern’s expert combos.
Jax Expert (5 points): Complete Jax’s expert combos.
Joker Expert (5 points): Complete Joker’s expert combos.
Kano Expert (5 points): Complete Kano’s expert combos.
Kitana Expert (5 points): Complete Kitana’s expert combos.
Lexluthor Expert (5 points): Complete Lexluthor’s expert combos.
Liukang Expert (5 points): Complete Liukang’s expert combos.
Scorpion Expert (5 points): Complete Scorpion’s expert combos.
Shangtsung Expert (5 points): Complete Shangtsung’s expert combos.
Shazam Expert (5 points): Complete Shazam’s expert combos.
Sonya Expert (5 points): Complete Sonya’s expert combos.
Subzero Expert (5 points): Complete Subzero’s expert combos.
Superman Expert (5 points): Complete Superman’s expert combos.
Wonderwoman Expert (5 points): Complete Wonderwoman’s expert combos.
Darkseid Expert (5 points): Complete Darkseid’s expert combos.
Shaokhan Expert (5 points): Complete Shaokhan’s expert combos.
Arcade Master (20 points): Beat Arcade Max without using a continue.
Training mode master: (50 points): Finish expert combo mode on MK and DC.
Special Moves Pro (100 points): Do all special moves including JF’s.
Online Chat Match (20 points): Win a chat room match Online.
Pugilist (10 points): Get all 4 hits in KK mode in an Online match.
Sway-zee (10 points): Counter a KK attack in an Online match.
Freefall (10 points): Do a freefall special move in an Online match.
Switch-A-Roo (10 points): Do a freefall switch-a-roo in an Online match.
Challenger (10 points): Play 10 Ranked Matches.
Online Champion (50 points): 25 Ranked Match wins in a row.
Competitor (50 points): Player 100 chat lobby matches.
Humiliation (20 points): Get a flawless victory Online.
Raiden Expert (5 points): Complete Raiden’s expert combos.

                                                          
Mortal Kombat Vs DC Universe moves + fatalities: 

LEX LUTHOR
 Target Practice - D, B + 1
 Powered Palms - D, F + 2
 Lexcorp Rocket - D, B + 3 (Also in air)
 Hot Flames - B, F + 3
 Evasive Manuever - D, B + 4
 Rocket Boots - B, F + 4
 Fatality 1 - D, F, D, B, 2
 Fatality 2 - U, U, U, 1

RAIDEN

 Vicinity Blast - D, B + 2 (Hold 2 to lengthen attack)
 Lightning Bolt - D, B + 1
 Projectile Cancel - D, F (Cancels Vicinity Blast & Lighting Bolt)
 Energy Teleport - D, B + 3
 Lightning Shock - F, F + 2
 The Superman - F, F + 4 (Also in air)
 Fatality 1 - D, F, D, U, 2
 Fatality 2 - B, F, F, D, 4

SCORPION

 Spear - B, B + 1
 Fiery Teleport - D, B + 3 (Also in air)
 Inner Flames - D, D + 2 (Hold 2 for longer flame)
 Hell Fire - D, B + 2
 Hellish Slide - D, B + 4
 Fatality 1 - D, D, D, 4
 Fatality 2 - F, F, B, D, 3

SHANG TSUNG

 Skull Fire - B, B + 2
 Sky Fire - D, B + 2
 Slide Launch - D, F + 4
 Soul Steal - D, B + 3
 Hot Escape - D, B + 4
 Body Switch - D, B, F + 1
 Fatality 1 - B, D, F, 3
 Fatality 2 - D, D, F, F, 3

GREEN LANTERN

 Summoned Hand Grip - D, B + 1
 Justice Grip - D, F + 3
 Strength of Will - D, B + 2
 Judgement Hammer - B, B + 3
 Wall Barrier - B, F + 4
 Heroic Brutality 1 - F, B, D, B, 3
 Heroic Brutality 2 - B, F, D, D, 4

JAX

 Gun Runner - B, B + 1
 Power Fist - D, B + 2 (Hit 1 or 3 after the punch for an extra hit)
 Blinding Light - D, B + 1
 Rocket Blast - D, B + 3
 Double Rocket Blast - D, B + 3, D, B + 4
 Gotcha Grab - F, F + 2 (Tap 2 for extra hits)
 Ground Pound - D, D + 4
 Fatality 1 - B, F, F, B, 2
 Fatality 2 - F, F, B, B, 3

THE JOKER

 Joker's Wild - D, B + 2
 Bombs Away - F, F + 4
 Bombs Away Close - F, F + 2
 Bombs Away Medium - F, F + 3
 Put It There Pal - D, B + 1
 Sinister Heels - B, F + 4
 Funnyman - B, D, F + 3
 Magic Trick - D, B + 4
 After Magic Trick:
 (1) Surprise Pistol Whip - 1 (2) Surprise Bomb - 2
 (3) Surprise Stomp - 3       (4) Surprise Slide - 4
 Fatality 1 - F, B, F, 1
 Fatality 2 - B, D, B, F, 2

KANO

 Kano Ball - B, F + 4
 Kano Up Ball - D, F + 2
 Brutal Throw - D, F + 1
 Knife Toss - D, B + 1
 Parry - D, B + 2
 Eye Laser - B, B + 1
 Fatality 1 - B, D, B, F, 1
 Fatality 2 - D, D, F, B, 4

KITANA

 Bladed Fans - F, F + 1 (Also in air)
 Square Wave Punch - D, B + 2
 Rolling Fury - D, F + 4
 Mystical Teleportation - D, B + 1 (Also in air)
 Fan Lift - B, B + 1
 Razor's Tip - D, F + 2
 Fatality 1 - B, B, F, 1
 Fatality 2 - F, D, D, B, 3

SONYA

 Ring of Doom - D, F + 1 (Can do twice)
 Deadly Kiss - D, B + 1
 Leg Grab - B, F + 4
 Bicycle Kick - D, B + 4 (Also in air)
 Kartwheel Flip - D, F + 3
 Square Wave Punch - D, B + 2
 Fatality 1 - D, D, B, F, 1
 Fatality 2 - D, B, F, F, 4

SUPERMAN

 Heat Vision - B, B + 1 (Also in air, Can be done 1+ times)
 Inhale Capture - D, B + 2
 Soaring Knockout - D, F + 2
 Ice Breath - D, F + 3
 Ground Tremor - D, D + 4
 Shoulder Charge - F, F + 4
 Up, Up and Away - D, B + 3
    After Up, Up and Away:
    1) Hover Heat Vision Close - 1
    2) Hover Heat Vision Far - 2
    3) Ground Tremor - 3 or 4
 Heroic Brutality 1 - D, D, F, F, 1
 Heroic Brutality 2 - F, F, D, B, 4

WONDER WOMAN

 Wonderous Spin - D, B + 4 (Also in air)
 Divine Princess - D, F + 2
 Splits Grab - D, F + 3
 Gotcha Girl - F, F + 4
 Handstand Burst - D, B + 3
 Lasso Grab - B, F + 1
 Heroic Brutality 1 - U, B, D, F, 1
 Heroic Brutality 2 - F, B, B, F, 4

SHAZEN

 Solomon Escape - D, B + 4
 Strength of Hercules - B, F+ 2
 Atlas Clap - D, B + 2
 Power of Zeus - D, B + 1
 Achilles Bolt - D, B + 3
 Mercury Bear Hug - B, F + 4
 Heroic Brutality 1 - D, F, B, F, 1
 Heroic Brutality 2 - D, B, D, F, 2

 BARAK

 Sharp Spark - D, A + 1 (hold 1 for delay)
 Scrape Kick - F, F + 2
 Chop Chop Blades - B, B + 3
 Blade Cyclone - D, F + 4
 Fatality 1 - B, F, D, F, 1
 Fatality 2 - F, F, D, D, 3

BATMAN

 Smoke Capsule - D, B + 2
 Batarang - D, F + 1 (Also in air)
 Leaping Shadow Kick - D, F + 4
 Leg Take Down - D, B + 4
 Sneaky Batarang - D, B + 1
 Dark Absorbtion - B, B + 2 (Projectile Resistance)
 Smoke Escape - D, B + 3
 Heroic Brutality 1 - D, B, F, F, 4
 Heroic Brutality 2 - D, D, D, U, 3

CATWOMAN

 Whip Sting - D, B + 1 (Also in air)
 Lasso Grip - B, F + 2
 Kitty Surprise - B, F + 3
 Raging Cat - B, F + 4
 Somersaulting Fever - D, F + 3
 Nine Lives - D, B + 4 (Projectile Resistance)
 Fatality 1 - D, B, D, F, 2
 Fatality 2 - F, B, F, B, 4

DEATHSTROKE

 Close Flash Bomb - D, D + 3
 Far Flash Bomb - D, D + 4
 Pistol Shot - B, F + 1
 Lunging Stab - B, F + 2
 Stomach Stab - D, F + 1
 Fatality 1 - F, F, D, F, 2
 Fatality 2 - F, D, B, F, 3

THE FLASH

 Fast Escape - D, B + 3
 Super Uppercut - D, B + 2
 Teleport Uppercut - D, B + 4
 Around The World - B, F + 4
 Flurry Punch - B, F + 2
 Teleport Flurry - D, B + 1
 Heroic Brutality 1 - B, B, F, F, 4
 Heroic Brutality 2 - D, F, D, B, D, F, 3