Press 'P' to pause the game and type in one of the following:
"SCREW"--Increases lives to 20
"GUN"-----------Infinite lives
"BOMB"------Skip to next level
"UFO"------------Invincibility
"ALIEN"-------------------????
Now press 'P' again to unpause the game and activate your chosen cheat mode.
If the above don't work try these while the game is paused just like above. There are slightly different releases of this game with one set or the other of these cheats working.
"CURRY AND RICE"----------Increases lives to 20
"WAIT DA MAN"--If you die, you wont lose a life
"BADBOY"---------------------Skip to next level
"DREAMZONE"-----------------------Invincibility