“How can I open a mswmm file so that I can watch the video?”
The short answer is you can’t just open an mswmm file and watch video because mswmm files are project files not video files. MSWMM is the project file format used by Windows Movie Maker to store information about which video clips, music, and photos are used in a video editing project. If you don’t have all those clips on your computer and you go to open the MSWMM file, the file won’t properly reference the elements of your video.
If you managed to lose the file association between Movie Maker and the .mswmm file extension, open Movie Maker, then go to open project and browse to the .mswmm file on your computer. This will open the MSWMM project in Movie Maker and should automatically find all the associated video clips, audio files, and photos used in your video project.
If someone sent you the MSWMM file thinking they were sending you a video, you will need to contact them and ask them to publish a movie from their video project. The video they send should probably be a WMV file, since that’s the standard format output from Windows Movie Maker.