This weekend I tried to add beat markers to a video for a colleague. But the beat marker menu option was grayed out, and I spent some time searching for the “Clip Trimmer” everybody was talking about. I was in a bit of a hurry and all the right-clicks, top menu, gear-icon popup menus, and shortcuts made me overlook the thing I was looking for. Maybe you overlooked it too. It’s not much of a tutorial, but if you need to add beat markers to your background music, here’s how to get to the correct “Clip Trimmer” in iMovie ’11.
Once you are in this trimmer, pressing “M” will set a beat marker right where the red playback cursor is. You can play the track and tap along, or you can aim carefully with your mouse. Have fun!