1. Open itunes
2. Go to Edit menu>Preferences>General
3. Find the part where it says 'use itunes for internet music playback' and click on 'Set'.
If that doesn't work, try opening your shared folder and dragging mp3s manually into the itunes window.
And if THAT doesn't work either, drag them over
itunes Associator first.
If NONE of these work then give up and trash the mp3.