To unlock these secret characters, just type in the code during the character
selection screen. Once you hear a sound that implies a correct code has been
registered, select the required character.

Effect            Code

Play as Brownie  - Press "RB RB RB B B B X" and select Meat Boy
                   (requires an Xbox 360 controller)
Play as Tofu Boy - ype "petaphile" and select Meat Boy.