Unlike Apple's mobile devices like the iPhone and iPad, computers need to be authorized to access songs and videos that you've purchased or rented through iTunes. While there's no limit to the number of mobile devices you can use with an iTunes account, Apple limits each iTunes account to five computers at any one time.
To sign in and authorize your computer with iTunes, you need access to the iTunes Menu Bar. Press. Press Ctrl-B, or click the small icon in the upper-left corner of the window and select Show Menu Bar.
Authorizing and Deauthorizing a Computer
Click the Store menu. If you haven't signed in yet, select Sign In and enter your Apple ID email address and password. Select Authorize This Computer and enter your credentials when prompted.
To deauthorize a computer, you need to go through iTunes on that computer. Click Deauthorize This Computer from the Store menu.