According to www.geethree.com, the company that offers 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/imovieHD6.html
The Slick line of software continues to work great with iMovie 6 (and versions 4 and 5), and are universal binaries to work on the latest Macs. These award-winning plug-ins allow you to create remarkable and memorable videos that will "Wow" your audiences and let your creative talents shine. Check out our 10 volumes that include over 1000 plug-ins.
If given the chance to create plug-ins for iMovie '08, we'll do our best in creating cool new transitions, effects and titles. If you would like to give feedback to Apple on this topic, you can provide feedback here:
http://www.apple.com/feedback/imovie.html
If iMovie has been completely rewritten, it is clear that they did not take into account compatibility with third-party plug-ins. Apple ought to indicate such incompatibility on the iMovie webpage to better inform their customers. Let's hope that Slick can quickly port their plug-ins to iMovie '08, and that Cupertino will help them to speed-up the process.
[translation by Linathael] JCJ