I believe you will need to add the song files to iTunes Music library before you can add them to iPod. *sighs* as described here: How to transfer music to my iPod (& iTunes) {click on link}

I no longer use an iPod & never used the sync function. But adding the songs to iTunes was always the first requirement before you could add them to an iPod. So I suspect this is still the case, but cannot recall the specifics of what Sync does.

Keep in mind the LimeWire playlist in iTunes is simply a playlist, the actual songs still remain in your LW download folder.