Before you click the button, when you hover your mouse cursor over the button to see what link it's going to, what does the link say?
I need all of you to send me a PM (if you haven't already) with your order numbers. Some other people have had this problem too and I'm not sure what's causing it. Digital River is going to look into it for us next week, and the more order numbers I can provide them to look at, the better.
In the meantime, download the game as described here
instead. You can send me your hardware fingerprint to request the activation key at the same time you send your order number.
EDIT: I didn't realize how widespread this problem is. Don't PM me. I am answering support emails right now. Request the activation key by emailing support. Thanks.