At the main menu, press F2 + F3. You should see the message "Cheats On".
Now hold + and press BackSpace for a Cheat Code prompt.
SCOUSER - Enable playtester keys
TURBO - Turbo mode
EVERY LEVEL SAVE - Can save after every level
ALLOW SINGLE - Single player on multi-level
PAPARAZZI - Press Insert to save a Screenshot (in C:\TEMP\PIX)
THE WHOLE OF CREATION - All cheats off
Playtester keys:
Shift-F9 - Skip level
F11 - Ammo cheat
F12 - Health cheat