Have you manually transferred files to the iTunes library, or are you trying to sync your iPod using the 'shared limewire tunes' which appears in iTunes?? That shared tunes is just a playlist & so you won't be able to use that to sync an iPod, create a playlist etc. Have a look at the link below, there's a lot of info & tips.
Transferring music to iPod & iTunes

If it turns out that you have been adding files to iTunes properly, tell us what happens when you try to sync your iPod. Do you see any error messages??