ok, what you want to do is fairly simple.
Step 1
lace the enemy where you want it to spawn.
Step 2:Right click the enemy to access its entity properties window, and type a name in the NAME box (would initially be whatever the entity is called in the library) You'll also want to set SPAWN AT START to no, and the SPAWN AFTER DELAY to yes, but don't give it a time. Also, make sure that MAX AT ANY TIME is at least 1, otherwise it won't spawn.
Step 3:create the trigger zone where you want it, then access its properties. Type the enemies name (the one you assigned it) into the zone's IF USED box exactly as it was entered in the enemy's name box.
Step 5:Set up scripts. Put plrinzoneactivateused.fpi as the zone's main script
That should do it