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
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