The Number of Allowed Machines Has Been Exceeded to Play a Subscription Song
You must deactivate a machine running one of the three active Yahoo! Music Unlimited accounts to use Y! Unlimited on an additional machine. You may use the Yahoo! Music Jukebox on any number of machines; however, a single Y! Unlimited subscription can only be used on up to three machines to download subscription music. Therefore, you must manually deactivate one of the machines if you wish to use Y! Unlimited subscription music on more than three.
To do this, go to one of the active computers and start the Yahoo! Music Jukebox. Be sure you are logged in with your Y! Unlimited subscription ID. Click File > Preferences > Y! Unlimited, and click Deactivate under Y! Unlimited Deactivation. You will then be able to access Y! Unlimited on another computer.