Go to 'Players' menu. Create a new player ('Add player' button below). Type your favorite definition for enemies
e.g. 'Monsters'. Then select created player in the list and click 'Dimplomacy relations'. Choose 'Enemy' radio button for the player named 'Self'. Then click OK -> OK:
http://img216.imagevenue.com/img.php?image=51880_make_mobs_the_enemy_122_51lo.JPGThen select the monster you like by clicking right mouse button and choose enemy player in the drop down list on the right:
http://img259.imagevenue.com/img.php?image=52182_make_mobs_the_enemy__122_136lo.JPGDon't forget save the map