syncing music to nanao

I'm trying to sync some new music on my new MacBookPro to an old Nano 3, but when I click on "music" in the header in the iTunes Library and then click "Syn Music" on the next screen, a message pops up that asks if I want to replace the contents of the iPod with the new contents on the computer. No, I don't. I just want to add the 10 new songs to the iPod. How do I get around this?