You enter codes at Barnett College. It's the 2nd door on the left in the main hallway.
You still have to buy characters unlocked this way, you just get access to them.
Cheats that are unlocked this way are ready to go, they can be turned on from the pause menu.
|FJUR31||Unlock Mola Ram (Gamestop Preorder)|
|VE029L||Unlock Belloq (Circuit City Preorder)|
|0P1TA5||Super Slap (Target Preorder / Chalkboard scene) starts with a Zero|
|VIES2R||Character Treasure (Best Buy Preorder) All Enemys Drop Studs|
|13NS01||Unlock Toht (Wal-Mart Preorder)|