At the main menu of X-Men enter SELECT, UP, L2, R1, L1, R2. You should hear a sound to confirm the code. Open Cerebro mode and everything should be available as well as the secret characters.
Cheat Mode Off
If you want everything in Cerebro mode to return to normal press and hold Start and Select at the main menu.
Behind the scenes movie pics
Finish the game with all ten characters (Arcade mode) in their movie costumes to unlock behind the scenes pictures from X-Men.
Play as Secret characters
To play as the bottom five characters in X-Men complete the game four times with a different character.
Movie costumes for characters
At the character selection screen press Square when picking your character to play in his/her movie costume.
Character FMV Ending
Finish the game with any character on any level to see their ending.
Historical Comic Pictures
Complete the game with every character in Arcade mode to access their historical comic book pics in Cerebro mode.
Character Comic Pics
Finish ten rounds in Survival mode with a character to open their character pictures in Cerebro mode.
Character Academy mode FMV ending
Finish Academy training with a character to unlock their corresponding FMV ending for Academy mode.
Finish Academy mode with all A's with any character to unlock their third outfit. Press O to pick it.
Character Movie Pics
Finish twenty rounds in Survival mode with a certain character in their movie costume to open their movie pics in Cerebro mode.
Finish twenty rounds with Gambit to open pictures from X-Men in Cerebro mode.