During gameplay, hold down TAB and hit the page-up key simultaneously.
Then let go and press the letter P. At the bottom of the screen it will
say "cheat enabled." This will make you indestructible. This won't help
on missions where you have to keep other things, like the War Drum, from
blowing up, but it will help in those nightmarish combat situations.
Cameron Lewis, Editor
ZENtropy! Electronics Reviews
Or you may try this alternate cheat:
goto offset 00000358
change that byte to 32 hexdecimal
save & exit.
You can now choose any mission you want.