this works for me.... i guess many people can't add some particular files, may be not all files
for my case, the mp3 files are actually some 'podcast' type files instead of 'music' type files for itunes, so itunes can never add the mp3 files into the music library, itunes just simply doesn't recognize the files as 'music' files!!
i only found one way to edit the 'media kind' which is in itunes
1. open the file with itunes in windows explorer (it can be done but the file won't showed up in the library only)
2. add the file into a playlist
3. go to the playlist
4. right click and 'Get info'
5. go to 'option' and you should find 'media kind'
6. if it is not 'music', change it to music
7. the file should automatically show up in your library
Good luck