Simply type these codes in during gameplay.
 dnallen:               says a message
 dnclip:                walk through walls (very unstable)
 dncosmo:               another message
 dnhyper:               steroids effect
 dnbeta:                yet another message
 dnskill#:              set skill level
 dncoords:              shows your coordinates
 dnrate:                displays the frame rate
 dnitems:               gives you power-ups
 dnkeys:                all keys
 dndebug:               displays debug info
 dninventory:           all power-ups
 dnweapons:             all weapons and full ammo
 dnmonsters:            toggles the monsters off
 dncashman:             throws money when you hit the use key
 dnview:                external Duke view (same as F7)
 dnscotty##:            level warp (episode, two-digit level number). To warp to level: 7 in episode 1, you would type dnscotty107. It is important to: include the zeroes 
  dnstuff:                     all weapons, keys and power-ups
  dnshowmap:               shows entire map
  dncornholio:             God mode and unlimited jetpack
  dnkroz:                  same as Cornholio, but easier to type