Finally learned how to install the trap door on the bin. REMEMBER - the bin and trap door pins are very delicate - treat them with a lot of care! Tools needed: long nose pointed pliers, bright light and patience. Insert one pin and gently compress the bin while using the pliers to help insert the other pin.
The trap door cost me $16.72 (including tax/shipping) but for $28/$30, you can buy the whole bin/trap door unit. So, if you get frustrated easily - best bet would be to buy the whole unit.