Go to Customize Player and enter these names. If entered correctly there will be a confirmation noise.
Cartman TAKE A DIVE
Gonzo HOTPOTATO
Gabo BIG HEADS
Kyle SKELETON
Mr Hat CRAZYBALL
Powder SILLYMOVES
Neila ALIEN - THIS IS COOL!
Zico Opens up first Classic Game
Hurst Opens up first four Classics Games
Kenny Flaming ball
Go back to Match Select and press Scroll Lock. Up comes the Cheats menu. You tick off the Cheats you want to activate - many of them can be combined for pretty bizarre effects.