On the title screen or during play, type "HAVE A BAD DAY". The screen should flash to let you know it worked. Now use the following keys:
'F1'-------Level 1 | 'F6'---Level 3 boss
'F2'--Level 1 boss | 'F7'--------Level 4
'F3'-------Level 2 | 'F8'---Level 4 boss
'F4'--Level 2 boss | 'F9'--------Level 5
'F5'-------Level 3 | 'F10'--Level 5 boss
'L'---------Advance to the next level
'D'--Kill all opponents on the screen