This happens with videos that are stored as referenced files in Aperture 3. Only managed videos are available in the Aperture Videos area of the iMovie Event Library.
To change referenced videos to managed:
- Highlight the affected videos in the Browser.
- Choose File > Consolidate Masters from the Aperture menu.
- In the dialog box, choose "Copy files" if you wish to leave a copy of the video in its current location, or choose "Move files" if you wish to remove the file from its current location. In either case, the video will be written to the Aperture library.