Press "P" and then BACKSPACE and write:

LAMER = Jump to the next stage
BOUF = God mode for a short period
CSTRIKE = Gives Jazz a flying skateboard
BAD = Gives Jazz a green bird
GUNHED = All weapons and 100 i ammo
SABLE = Jazz runs faster
DOOM = Makes the game harder
KEN = Close Jazz Jackrabbit
HOCUS = Moves you to different locations
ARJAN = It will stand HAHAHA and the screen starts to shake
MARK = You will die
GREETZ = Mess: Hello, world!
TIM = Mess: Awesome
CHECK = It will stand some numbers on the points

If it´s not working try to put DD before the code. Ex: DDBOUF.