Q: how do i authorise my computer to watch downloaded movies?

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

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

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Open iTunes and go to the Store menu (in the menu bar) -> Authorize this computer. Then, try watching your iTunes movies.