In LW tools > options > iTunes > sharing > make sure that 'share my music' is ticked > apply > ok.

Some versions of LW/iTunes didn't show me a shared 'LimeWire Tunes' either, I don't know why . iTunes6 was fine, but once I switched to iTunes7, I never saw the shared tunes at all...until I used the latest LW Beta version & now it's back So, if you've recently changed LW or iTunes versions, maybe that's the issue.

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.
Vista users: LimeWire Pref Folder in Vista