There's three possible problems:
1. There are spaces in the file name. If there are, replace them with underscores (_).
2. The track is stereo. If it is, it should look like it has two tracks in audacity. Simply go to Tracks > Stereo to Mono to fix.
3. You're placing them in add-ons, and need to place them in add-ons/music. I don't think this is the case because you don't seem like an idiot, but you didn't specify that in the op so I just wanted to make sure.