According to www.geethree.com, the company offering plug-ins for iMovie 6, iMovie 7 (aka iMovie '08) does not support their plug-ins anymore, and it does seem to be a mistake from Apple:
iMovie '08 - Message from the Slick Team
iMovie '08 is a complete rewrite of Apple's movie editing software and offers a new model for creating movies. It sports a new look and provides a fast and easy way create a basic video quickly. Unfortunately, this new version does not support plug-ins such as the Slick software.
For your video projects that involve more than a standard dissolve transition, iMovie 6 lives on! When you install iLife '08, a copy of iMovie 6 stays on your system. And if you purchase a new Mac with iMovie '08, you can get a free download of iMovie 6 here:
http://www.apple.com/support/downloads/imo
