If you have an iPhone the ability to sync Address Book on your Mac is enabled in the preferences once the iPhone is synched for the fisrt time. So how do you sync your Gmail contacts with Address Book if you don’t have an iPhone? Simple.
Open up Terminal and type the following followed by the enter key:
defaults write com.apple.iPod Devices -dict-add red-herring ‘{ “Family ID” = 10001; }’
That’s it! Now open up Address Book go to Preferences and you will see Sync with Gmail in the sync options right under Yahoo!