Sync contacts with mountain lion mac contact list
This is explained in detail on the yahoo! help page.

I'll post the original text from the help page here:


  Description
  
  This article explains how to synchronize your Mac OS Address Book app to Yahoo! Contacts on your Mac for versions of Mac OS X 10.5 ("Leopard") and newer.
  
  Resolution
  
  You can sync Yahoo! Contacts with Mac Address Book.
  
  Note: Only Mac OS X 10.5 ("Leopard") and newer support the sync feature. To check what version of Mac OS X you've got, select the Apple | About This Mac.
  
  How to sync your contacts with Mac Address Book:
  
  
  Open the Mac OS Address Book app.
  Go to Address Book | Preferences.
  Select Accounts | On My Mac.
  Check the checkbox for Synchronize with Yahoo!.
  Tap Agree for the Terms of Service.
  Type your Yahoo! ID in the Yahoo! ID box.
  Type your password in the Password box.
  Tap OK.