Q: How can I get my address book from my iMac (Mountain LION) on my iPod nano

What model iPod do you have? The 6G Nano does not support Contacts or Calendars.

Identifying iPod models

B-rock

Several users running Mac OS X Lion and now ML (and iCloud) have no longer been able to "sync" contacts over to their older iPods models.

A work around is to export all of your contacts out of Address Book as vCards and them copy these vCards into your iPod's Contacts folder.  You'll need to have the Enable Disk Use option ticked from under the iPod's Summary tab in order to browse out to the iPod's Contacts folder via the Finder.

Using your iPod as a storage drive

That's the only "solution" I have been able to use to get contacts to sync/appear on older iPod models. 

B-rock