You'll want to purchase two locks to fully secure your metal security / bear box.
1. You'll want to lock the two parts of the metal box together using a 5/16" or 1/4" pad lock
2. This python cable lock (or similar) to lock the box to a tree or pole. In leiu of this lock, you could screw the box to a tree or wall using our pre-drilled holes in the back of the box.
If you're unsure how to close the box or place your camera inside, this video might help.