I have iMovie '11 and wish to export an iMovie project to a mov file that I can import into another iMovie project. How can I do this?
MacOS – Export an iMovie project to QuickTime to import into another iMovie project
imoviemacos
Related Solutions
Just to echo what another user said in a comment, if installing Perian and DivX didn't work you might try converting the video. Just download Handbrake and convert your video to H.264, then try importing that into iMovie.
Make sure that it shows nothing over here: 
under today's date or the title that you have given it. If there is indeed nothing, then try File > Update Projects and Events. If it still doesn't show up, then your only hope is that you have a time machine and that you backed it up.
Just asking, what version of iMovie are you using?
Best Answer
Open the project, then select Share → Export Movie.
Select a destination and other options if relevant, then select Export. Once the project is exported, you can import it just like any other video.
More information: iMovie ‘11: Export your movies to a Finder folder