The unlock code is your name as it appears on your Credit Card, and the Credit Card number.
Alternatively you can download Nook for PC from B&N: http://www.barnesandnoble.com/u/nook-for-pc/379003591/
Or if you have Windows 8 you can use the Windows 8 application: http://www.barnesandnoble.com/u/nook-for-windows-8/379003757/
B&N has been moving steadily towards a 'captive state' relationship with
its Nooks. They may be requiring an 'unlock code' from you now, and I'd
believe that code is stored on the Nook.... And, since the Nook won't
work, I'd call B&N and discuss this 'unlock code' with their technical
support folks. Maybe they have a work around for PC's.
No, this has been the case since ADE updated to 2.0 and finally started supporting B&N content like Adobe said they would 3 years ago. (the question was how to read Nook content in ADE - as opposed to the more usual question of how to read ADE content on the Nook).