From what I can tell, a hacked Flash plug-in will get you closer to your goal, but won't let you accomplish what you want.

Amazon will show the VOD purchase screen, and I "bought" some sort of free "cast talks" video from the page to which you linked. However, the large Flash area near the top of the page--a section I take to be the actual player for the video--never progresses beyond showing "Loading" and some animated dots.

I've no prior experience with Amazon's VOD service, but from what I can tell, the "Download" button you want is part of that player that never loads. Therefore, you won't be able to get the videos you purchase with a hacked Flash plug-in.