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.