Only I (the Admin) can download mp3 files. I tracked the problem down to two bugs. Both reproducable on three different Macs.

Bug 1
1. log into a non Admin user
2. open options dialog
3. set the download save location to the iTunes Music folder located in the users Document folder
4. save the options dialog
5. quick LimeWire
6. Open LimeWire
7. open options dialog
8. the download save location you set in step 3 wasn't saved

bug 2
You can only download mp3 files if the download save loaction is in a users local folder as in step 3 above. The shared folder in LimeWire will NOT work. Because of bug 1 and this bug, the only way to download files is to reset the save loacation every time you start LimeWire.