What have you tried so far in terms of troubleshooting this issue? Are you plugging the iPod into a high powered USB 2.0 port on the back of your Mac? Have you tried a different USB cable?
What happens if you try to reset the device with it still connected to the Mac?
Has this iPod ever worked on this Mac or is this the first time you have time you have tried connecting it?
Have you carefully worked through each and every single suggestion in this Apple support document?