O.K. there are MANY ways to pull videos from YouTube and I have tried quite a few. This one seems the easiest, to me, and it quite configurable if you do a lot of this type of thing.
Step One: Download and install FireFox
Step Two: Install a Firefox Plug-in called Video Download Helper
Step Three: If an MP4 version of the video exists. Download that and import to iMovie
DONE!!! I told you it was easy.
If the only version available for download is an .FLV file grab that file and covert with MPEG Streamclip BETA (not the regular one). MPEG Streamclip will need the Perian Quicktime components. When you open the file in MPEG Streamclip just export out Apple Intermediate Codec at your project frame size, full quality. Then import that file to iMovie.