Pause the game and type "ZOBINETTE". The screen will flash to let you know the cheat is activated. Now unpause the game and press any of the following keys.
'M'---------------------------For extra lives
'N'------------------------Skip to next level
'DEL'-------------Kills all enemies on screen
'CTRL'--Kills the big boss in the bonus stage
If the above doesn't work try "JC IS THE BEST" in the same place.