You may encounter a problem where iMovie will not display movies in the iMovie "Event Library" browser if they were were imported with iPhoto. While movies created in other applications or that you otherwise have on disk may work, those that you have stored and managed in iPhoto will not show even when you click the "iPhoto Videos" section in the iMovie Event Library.
One of the convenient features of iLife applications such as iMovie is being able to easily grab relevant multimedia files that you have imported into other iLife applications. Unfortunately sometimes application-specific settings may interfere with this process, and this seems to be the case with this problem.
To fix this problem, turn off music for existing slideshows in your system. A number of people (such as those in this Apple discussion thread) have found doing so enables iPhoto videos again in the iMovie Event Library.
Optionally, you might try accessing the movie file directly instead of using the Event Library. To do this, open iPhoto and locate the desired movie. Then right-click it and chose the option to "Show File," which should highlight the file in the Finder. Then go to iMovie and right-click in the "Event Library" area, and choose the "Import Movies..." option.
An "Open" dialogue box will present itself, and you can drag the previously located movie file to the "Open" window, and you should see it appear in the window as a selected item. Click "Import" and it will be copied to the current iMovie project as a new event (or appended to an existing event if you so choose). While this will work, keep in mind that it will not be using the movie that is stored in the iPhoto library, and instead will be using the copy that was made.