Sometimes, you end up with videos in weird containers, and your video editing program of choice just won't open them. That's okay though, usually the video itself is just fine, and the program just doesn't support the container in question. With MeGUI, you can easily extract the raw video and audio data, and put them in a more conventional container like MP4.
Go over to Tools -> HD Streams Extractor. Here you can grab the raw data streams from your video file.
After you've extracted them, go over to Tools -> Muxer -> MP4 Muxer, and select the files you've just extracted. And that should be it!