CardDAV access to Zimbra under Mac OS X 10.6 (Snow Leopard) is broken on the Apple's side. Don't waste time on it as it won't work.
Running 10.7 Lion though it should be pretty straight forward:
- Open System Preferences
- Go to 'Accounts'
- Select 'other account' at the bottom of the list
- select 'CardDAV' account
- enter user name, password and the hostname of your server
This should create a new account in Address Book with everything defined properly.