Make sure you have the latest version of CompositeLab installed:http://fxhome.com/account/programs
There was an update not that long ago to improve Quicktime compatibility with iMovie. Do you know what codec iMovie is exporting the files into?
On a related note, if you plan on making movies for some time, I encourage you to get as far away from iMovie as quickly as you can. It is among the worst video editors out there. Unfortunately, there aren't may options for inexpensive editors that are decent, on a Mac. Final Cut Express is your best bet, but it's not exactly cheap.