Ok, here's how it works:
1. Pull or taunt (1) monkey away from all the others,,, fare enough they don't become 'excited' by the yowls of a trapped monkey!
2. "examine" the trap in your inventory to get the pop-up screen that says "place the trap". I dragged my trap to my hot-bar so all i had to do was double-click it to get the window to pop up.
3. place the trap, and the monkey automatically goes into it.
4. click on the trap and select 'pick-up'
5. take the now trapped monkey to the npc and turn him in. you automatically get another trap.
this should help clear up the procedure.
Jeaack