Hi, first, please make sure the video could be compatible with windows movie maker.
Here are Supportable formats for windows movie maker:
Video files: .asf, .avi, .wmv
Movie files: .mpeg1, .mpeg, .mpg, .m1v, .mp2
Audio files: .wav, .snd, .au, .aif, .aifc, .aiff
Windows Media files: .asf, .wm, .wma, .wmv
Still images: .bmp, .jpg, .jpeg, .jpe, .jfif, .gif
MP3 format audio: .mp3
Second, as f ar as i know, most flip videos are in .mp4 foramt which could not be recognized by windows movie maker. That is to say, you should try a flip video converter to convert it to .wmv which works well on windows movie maker. Here i wanna recommedn SnowFox Total Video Converter which im using now. it works great for me. and many of my friends are using and in favor of it, not only because of its powerful conversion, but also of its superior video quality.
Good luck to you!