If you’re a Mac user that lives and dies by your Address Book, you’ll love this tool. One of our developers wrote a plug-in for the Mac Address Book application that lets you start a call on Yahoo! Messenger right from your friend’s contact card. Once the script is installed, you can right click on any contact’s phone number and you’ll see an option for “Call with Yahoo! Messenger”.
Here’s how to install the script which will work in conjunction with Yahoo! Messenger for Mac 3.0 Beta 4:
1) Download the zip file and unzip the contents (1 file entitled “Yahoo! Messenger.scpt”)
2) Double click the hard drive icon on your desktop, go to the Library folder, and find the folder entitled “Address Book Plug-ins” (~/Library/Address Book Plug-Ins/)
3) Drag and drop the Yahoo! Messenger.scpt file you downloaded into the Address Book Plug-ins folder. (You may be asked to authenticate the installation)
4) After restarting the Address Book, you will be able to right-click on anyone’s phone number and call them through Yahoo! Messenger.
Source : http://www.ymessengerblog.com/