Assuming that you have got the computer activated and the books downloaded to the computer:
The Nook may have got half activated. In that case, deactivate it using ctrl-shift-E while in Library view ADE with the Nook plugged in.
Then restart ADE (maybe not needed) and try to activate the Nook again.
If it is the computer that won't activate, similar story.
Use ctrl-shift-D in ADE library view to deactivate the computer. Restart ADE and reactivate the computer.
If you still have issues, try using the older more reliable ADE 1.7.2 instead.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Thank you so much! You're a true lifesave to my stress levels and hair. You're suggestion was wonderful!!!!
Excellent. Could you please let us know which bit worked for you, it may help others in future.