"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.