Piece of cake. To install Yahoo! Messenger with Voice Beta:

1. Close all running Applications except for your Internet browser.
2. Visit the Yahoo! Messenger with Voice download page.
3. Click the "Get Started" button

Note: Download time can take anywhere from 1 to 15 minutes. You need 8MBs of free space to install Yahoo! Messenger.

After installing, launch Yahoo! Messenger with Voice Beta. If you have a Yahoo! ID and password already (that you use for Yahoo! Mail, Yahoo! Photos, Yahoo! Games, etc.), you can enter it. Otherwise, click the "Get Yahoo! ID" button to create a free Yahoo! Account.