This video tutorial explains how to use Handbrake to convert MP4 files into M4V files without buying expensive conversion software. If your camera shoots MP4 files, you’ll have to convert them before editing them into iMovie 11 because iMovie 11 only understands an MP4’s audio, not its video. (I’m hoping Apple will change this in future versions of iMovie.)
iMovie 11 WILL import the MP4 files, but you will only get audio, not video. This is evident right away in iMovie when you try to preview your clips in the event library. You will hear the sounds but only see a black screen. This tutorial will help you solve the problem without spending any money.