I tried importing a few different flavors of mp4 into WMM and none of them would go. This is on a machine running XP SP3. That led to a google search, confirming mp4 as a format *not* supported by WMM. I'm not sure how you were able to get the video to import, but more power to ya.
You can convert the file into something that WMM recognizes, like avi, wmv, or MPEG-2. The drag is that when you transcode from one lossy format to another, there's a quality hit. If you're not intending to play it on a 65" 1080p screen, then it may not matter.
There are free apps on the web to do this, but they often come from "suspicious" sites. The VLC player has some limited transcoding ability. If you have access to a Mac, iMovie will let you edit and output another mp4.
Wish I could be more help, but hope this gets you started.