Can't rent/buy a movie in iTunes

I've been interested in renting or buying a couple of movies recently. I searched for them in iTunes on my iMac. They showed up in the search results, but they buy and rent buttons were greyed out. I've seen this before where the buy buttons were available but the rent buttons were not, but that usually is only when they first come out, and then the rent buttons become available a month or so later. In this case, both buy and rent buttons are greyed out.

I thought this was somewhat strange, but figured they were just letting us know that they weren't quite available yet, but would be soon. I recently bought an AppleTV, and while playing around with it I realized I could buy and rent it via the AppleTV (well the buttons were available, but I didn't actually try). So, I went back to my iMac and they were still greyed out. I've also checked on my iPhone and I appear to be able to buy and rent it from my iPhone as well.

So, my question is what setting etc. might be out of whack on my iMac? I can't imagine why this would be considered on purpose, so I'm assuming something is wrong with my iMac.

I have a recently upgraded iMac running Moutain Lion 10.8.0. My itunes library is on an external USB connected drive, b