I had a lot of trouble importing .m3u playlists into MusicBee. (Windows 7)
Whether it was via rescanning files or manually adding them to the app info folder, nothing was working.
I worked through all of the different processes I could find in this forum and still no joy.
Eventually, I made MusicBee my default program for .m3u files. I opened them from Windows explorer and they started playing.
By choosing the Now Playing playlist, I could see my playlist. Then I just had to right click Now Playing and go to List > Save as Library Playlist.
Hope this saves someone a bit of frustration.