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?
This is the first time I am trying to use the Ipod 5th generation on the Imac, we were trying to use it on a PC before, but we had the same problem for some time that the ipod keeps saying : "use Itunes to restore" but itunes on the pc wouldn't recognise it. We have no problem with itunes as it works with the ipads and iphones