Go to Extras -> Cheats and enter the following combinations to get the desired results. After that, click on the star to mark them for use in the game.
Code - Result:
xxywywz - Super Health
ywxzyzxw - Unlimited Ammo
zxzxzxzx - Unlimited Grenades
wwwwwwww - Wacky Deaths
yxzwzxyw - Director's Cut
zzzzzzzz - Bouncy Grenades
xxxxxxxx - Zombie Walk
yyyyyyyy - Fire Bullets
zyzyxxww - Rapid Rails
xxxxyzxx - Instagib Ammo
wzxywzxy - Instagib Explosions
zzzzwxzz - Bullet Gibs
wwxxyzyz - Unlock Everything *
yzwzyxwx - Unlock All Cheats *
zyxwyzxx - Unlock All Levels *

* denotes that all other codes must have been entered before these can be used.