I also had this problem today, only on http://www.hbodigitalhd.com/#redeem where it says:
1. Click below to launch iTunes.
2. Sign up for an account or log in.
3. Download and enjoy your digital copy.
The "Launch iTunes" link it provides only takes me to: http://www.apple.com/itunes/download/
This page does not provide me an opportunity to login to iTunes, but rather only asks me to re-download the iTunes client.
I eventually did this, but it did nothing for helping my download make itself available to me.
I put a support ticket into the HBO site, but I'm uncertain which end (HBO or iTunes) the problem is actually occurring at.
Has anyone been able to resolve this?