Could you post more pics of your terrarium, Maddie? That might help out as there are several different types of terrariums.
From what I've seen of your other pics, you don't have the kind that needs the "lock" I see (also in your pics). It seems that you have a metal lid with metal mesh across the top. You'll need locking "clips" for that one.
The type of terrarium your lock goes to has a sliding lid that has a hole that matches a plastic hole on the actual terrarium as well. On this, you close the lid, thread the lock through both holes and viola... locked lid.
I might have seen things wrong, so more pics would definitely help.