On your Computer...
If you're using iTunes 11 click on the box icon in the top-left corner to see the " Show Menu Bar "
and from the Menu Bar select Store...
From the Store menu, choose Authorize This Computer.
(In earlier versions of iTunes, access this option from the Advanced menu).
When prompted, enter your Apple ID and password, then click Authorize.
See Here for full details...
Open itunes. On the upper left corner of itunes (Menu Bar), Choose Store > Authorize This Computer.
Hope this helps.
Finally, someone who could help! Thank you, Guardian Angels. I looked EVERYWHERE last night and everyone is telling me to click on the "store" menu, but I couldn't find it. Your one bit of info about clicking on the little square to open the menu bar was left out of the instructions on the Apple website and all of the blogs I've looked at. You would think that Apple could include that tiny tidbit of information. Thank you!!!!!!!!!!!!!!!!!!!!