Hi Andrew,
That "Limewire Tunes" is just a playlist, so you won't be able to add to iTunes or iPod from there. That playlist doesn't contain the music files, rather it just points to where they are on your computer
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Add to iTunes from your saved folder instead. In FW's library click on Saved Files then click Explore...your saved folder will open & you should be able to add to iTunes from there. There's a lot more info here (things are the same for FW as they are for LimeWire).
Transferring music to iPod & iTunes