The shared 'LimeWire Tunes' thing in iTunes is just a playlist...doesn't contain music files. You'll need to find your saved folder (where your LW downloads are saved) & then add to your iTunes library from that saved folder

In LW's library, highlight a file & then click Explore...your saved folder will open. Add to iTunes library from here. Once those files are in your iTunes library, you'll be able to add them to your iPod.

There are a few ways you can add to your iTunes library, dragging from the saved folder into iTunes works for most people but some people find that doesn't work. If you use iTunes to play a song in your saved folder, that song will be added to the iTunes library. In iTunes, if you click file > add file to library > browse for your saved folder, you can add that way. Or file > add folder to library will let you add the whole folder at once. Have a read through this thread for more info
Transferring music to iPod & iTunes