Go with the cursor to the 'Hints for Videogames' book in the inventory and type in:

 QUICKSUCKITOUT              Poisons cannot harm you
 YOULOOKINATME               Unlimted health
 IFEELUNUSUAL                Unlimted ammo
 PATRICKMARLEY               Listen to Darnley Microcorder
 ILOVETHISGAMEWHOMADEIT      Credits