iMovie HD Missing Sound Effects
I was experiencing the same thing and going crazy trying to figure out how to import them into iMovie. I didn't figure out how to do it the regular way, but did find a way to use them easily.
1. open a new project in iMovie
2. open your folder where the iLife Sound Effects are
3. open the first one: Ambience and drag all those files into the audio part of the time line in iMovie.
4. Create another folder on your desktop and give it folders with it with the same names as the iLife folders (i.e.: Ambience, Animals, etc)
5. Now drag the files from the audio time line into the NEW folder (it will become an aiff file)
6. Once you have done this with each and every folder, you can then drag each folder separately into iTunes (which will convert them to mp3 files) and you will be able to easily see them and use them in iMovie! You can also add any other .caf (core audio file) files this way.
I can rest now...