There are many people having issues syncing with iPods, particularly the iPod classic. I had the same issue and searched the various forums and found nothing but people asking for help. In the end I got my iPods to sync but doing the following, I do not know if this works for everyone and I do not know if you need to do all the steps, but this is what I did.
- Close down iTunes.
- Hard Reset your iPod - Press and hold the menu and centre button until it re-boots.
- Once the iPod is re-booted plug it into your Mac via USB without opening iTunes first, iTunes should just open itself with out your intervention.
- Now your iPod should appear in iTunes, well it did for me anyway.
Hope this helps.
Pete