1) Please download the file "MPEGLayer3.zip" from here and extract it to a temporary location.
2) Upon extracting, please copy the following files to the "C:\Windows\System32" directory. If you are prompted to overwrite any files, please do so.
3) After copying the files, execute the file "mp3reg.bat" which will register these files on your machine.
4) Lastly, launch your muvee application to verify whether you can import MP3 files from your computer.