Go to the main menu, hold LT, RT, LB, and RB and press Down, Up, Yellow, Blue, Down, Down, Yellow, Yellow and a new option will appear under Extras named Cheat. Now select Cheat and you will see a text window where you can enter the following codes.
all levels: KKst0ry
all extras: KKmuseum
healing for Jack: 8wonder
one-hit kills for bullets: GrosBras
999 bullets: KK 999 mun
500 rounds for Thomspon Machine Gun: KKcapone
50 rounds for the Sniper Rifle: KKsn1per
unlimited spears: Lance 1nf