Open your new iMovie project. Name it, etc. Go to the youtube page of the video you want to download. Before the video finishes loading (this makes it much easier to identify) double click the link on the window that pops up that has BLANK of BLANK MB to the far right of it, where all the other mb's and kb's and so on are listed. Once this finishes downloading, it should be saved to your desktop as get_video. Add the extension .flv to the end of it. Drag this into your iMovie presentation.
Voila. It would be easier to figure out to use a program, but eventually you get used to this and its MUCH faster.