Here's a good post from over at the Xbox Forums. It has a moves list, VS screen codes, the codes for the secret characters and more....
http://forums.xbox.com/7609212/ShowPost.aspx
The easiest way to unlock the secret characters is to load up a single player game, choose the hardest difficulty ladder and lose to the CPU. Let the continue timer run out and then input the codes when the prompt comes up. You'll need to do this 3 times (for each of the 3 characters).
You have to have 2 controllers to enter the codes. It helps to have a friend assist you but it's not impossible to pull it off yourself if you're quick. Mileena and Classic Sub-Zero are the easiest to do yourself (same button press layout on each controller). Ermac is a bit trickier (different set of button presses on each controller).
Just be warned that there is currently a bug in the game regarding the secret characters and the screen size setting. They won't stay set permanently at this time. The first time I had them unlocked, they stayed up for about 24 hours before I lost them and had to re-enter the codes again.
Good luck....
