In this video from macmostvideo we learn how to import unsupported video in iMovie. This happens sometimes when you buy a camera that is not compatible with iMovie. You can add certain video formats to your Mac to make your Mac understand these video formats that don't work with your iMovie. A common one is called mpeg2. You can get this trough Apple. In other cases you get the software with the camera. If you need to convert a video file, try to open it through QuickTime first. If that works, open it and save it as a different format. Another way is to use the VLC video player. You can download this. You can go to File, Streaming Export Wizard, set it to Transcode/save to file, select the file that is open and set it to Transcode video and audio, and set it to H.264 for the video and mpeg4 for the audio. Export this as and mpeg4 file. Another program is ffmpegx. You can drag and drop videos into it and set it to export and encode it. Another program is mpeg streamclip. Again you can bring the program in and export it in various formats. You can even preset your settings on this.