B&N is doing some general server maintenance today that they warned could affect the apps and downloads.
You may just want to try again tomorrow.
Are you running on an unmodified NookColor or rooted? If your nook is unmodified and having this problem, and it wont work today, I'd call customer service immediately to try and get it worked out. They've gone to a lot of trouble to make sure apps worked before they let them on the site, that type error shouldnt be happening.
I couldn't run any apps on my new Nook Color (I was getting the "Unable to Download..." error too) until I manually installed a software update. I know this thread is old but it's one of the references that turned up in web searches as I tried to figure out the problem. For whatever it's worth, I wrote more about this: Nook Color: Unable to Download Apps.