How do you use custom characters?

You can only use them in Free Play Mode, and apparently only one at a time. When you are choosing a character for Free Play Mode, press L button (or cycle the long way, but it's in between the first set of 5 chars (Superman, Batman, Robin, Nightwing, and Batgirl) and the last set (LexBot and goons)) There will be 3 characters with ? instead of their face, and these are your custom characters so you can choose one to play. The other 7 chars seems to be automatically chosen by the game.