How do I get a refund if the movie I rented cannot be played? (due to iTunes "unknown error")

I'm not sure whether rentals appear on your purchase history, but try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that film and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, about 24 hours).

If it doesn't appear there (or if that process doesn't work, it's been taking some people to this site on a browser instead of showing a form in iTunes), then try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.