Why can't I add background music to iMovie 9.0.9?

I've never used iMovie before, but I can't figure out how to add music. I've tried dragging the selected track from the sidebar to the clips but it just snaps right back into the sidebar. The videos I've found haven't been helpful because there's no "gray space" in the background to drag the clip to, and I don't have a bar for background music under my waveforms. Please help!