What version of iTunes are you using? There are compatibility problems with the 'shared limewire tunes' in iTunes 8, 7.6 & 7.7.

That shared limewire tunes is just a playlist (songs aren't actually in the iTunes library), so if you add files to the iTunes library - they should show in the library & be playable there. Check out the thread below for more info
How to transfer music to my iPod (& iTunes)

If you're interested in reverting to an older version of iTunes, to avoid these compatibility problems, see this post: http://www.gnutellaforums.com/tips-t...tml#post327806