If your iPod previously used on a Windows computer isn’t syncing with a Mac, you need to check the Disk Format on your iPod.
On iTunes, go to Devices and click on the iPod. It will display the Name, Capacity, Software Version, Serial Number and Format. The format can be PC or Macintosh and needs to be equal to the computer you’re using.
To change the Disk Format on your iPod, click on the button Restore