Q: how can I authorized my computer

For a computer running Mac OS X, move the cursor to the very top of the computer's screen, click on Store or Account in the menu bar, and choose Authorize this Computer.

For a computer running Windows, click here, follow the instructions, click on Store or Account in the menu bar, and choose Authorize this Computer.

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iTunes Account menu item > Authorize this computer on the computer you want to authorize.  Make sure iCloud is enabled.  iTunes menu item > General check icloud box.  To place the songs or albums on your computer you will have to click the three red dots and choose make available offline.