This post is several months old but I experienced the same issue with Unbox when I migrated over to Win8/8.1. Here is post from the Amazon forums that solved the issue of getting it to work. All credit goes to T.A.Clark for solving the issue.
The original post can be found here:
http://www.amazon.com/gp/help/custom...F7JZC&cdPage=2
Alright, I think I found a solution if you're still around Robert:
You're going to need this link:
http://drmlicense.one.microsoft.com/Indivsite/en/indivit.asp?force=1
You'll need to run it in Internet Explorer; it won't work in Chrome or Firefox. You'll also need to make sure and run Internet Explorer as an Administrator or the tool will give you an error instead of doing what you need it to. You can do that by right-clicking the IE charm on the Start screen (not desktop view) in Windows 8 and selecting the option to run as administrator on the bottom bar.
Run the tool at that link in the IE window you opened, and it should fix the issue after it completes (make sure and allow any system changes it tries, you might get 1 or 2 notices from Windows). Try Unbox again afterward and see if it works.
It seems to have just been a DRM issue causing the problem after a Windows 8 upgrade. It's likely that the user in Jonathan's post never noticed it because he/she had an OEM or clean install of Windows 8, but this is going to be very common for people who are doing a direct upgrade from Windows 7 to Windows 8 and are trying to get Unbox to work afterward. The more frustrating part of it is that it doesn't actually LOOK like a DRM problem when you're trying to get it to work...old videos played just fine, and the main issue was with downloading new videos rather than playing them.
I'm able to download again after doing this fix, but Unbox does still have an issue in the Troubleshooter where the notification service doesn't work on Windows 8...it just doesn't seem like that's the cause of, well, anything oddly enough. Either way, this DRM fix seems to correct the download problem.
Good luck, I hope this fixes the problem for anyone else having the issue.