On the main menu, highlight Extra Content and press R. On the password entry pad, enter the passwords bellow to unlock their perspective Cheat. If you entered the password correctly, you'll see a confirming message at the main menu screen. Cheats can be selected on the Extra Content Menu.

Cheat - Password
All Powers - 537893
Bullet Resistant - 548975
No Head - 987978
Super Psi - 456456
Unlimited Ammo - 978945