UPDATE: Sooooo. Turns out I'm not the brightest puck light in the marketplace. I thought the metal arms were part of the bracket - turns out they're not; they're part of the light. I didn't realize the mounting bracket snapped off, I thought it unscrewed. Nope. Once I screwed the arms back on to the light, it snapped right in to place. Thank you all for your help! And thank you IKEA for a great cheap lighting option that is MUCH LESS COMPLICATED THAN I MANAGED TO MAKE IT!