I have installed springs on a number of cap boxes and patch poxes in the past, but not on one with the hinge running fore-and-aft, so to speak. I think they intend to lay the spring flat on the bottom of the cap box recess, aligned vertically, or transverse to the stock direction, The bent end will be down, and will bear on the cam cast into the bottom of the door. The spring will therefore keep the door shut, but also will hold it open. This is a function of the shape of the cam on the door. Play with it for a while so you understand how it is supposed to work before installing it. If you still can't figure it out, bore a big hole in a piece of pine, screw the cap box down over the hole, inletting for the door cam, and practice with the spring until you understand how it works. It is not particularly intuitive.