In LW tools > options > iTunes & make sure that you've got 'share my music' ticked.

You could also try deleting LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will change all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.
In case you're using Vista: LimeWire Pref Folder in Vista

Have you updated/changed your version of iTunes? I used to never see the shared 'LimeWire Tunes' when using iTunes 7 (iTunes 6 always showed the shared playlist)...but after downloading the LW 4.15.0 Beta, the shared playlist magically appeared again in iTunes 7