Have you registered ADE the new machine with the same AdobeID as you were using on the old machine, and are presumably using on the Nook if library books used to work?
You may need to deregister whatever is currently registered on your new ADE, using ctrl-shift-D (cmd-shift-D on Mac). Restart ADE and register with your old ID.
thanks so much - I FINALLY figured it out and am happily reading again. Neither Barnes & Noble nor Adobe were of any help to me. I ended up using an alternate email address and used hubby's name instead of mine.