I'm in the same boat.
I already had an Adobe ID.
After registering my Nook, I activated my Mac with ADE. I downloaded some books on the ADE. All went smoothly there. So, I attached the USB from the Nook to the Mac and waited to see the Nook in the library. Never did. After closing out the ADE, I ejected the Nook from the Mac and unplugged the USB from the computer. Then, I plugged the USB in again and opened the ADE and waited for Nook to show in the library. Didn't this time either.So, I went downstairs to the kid's Windows computer, dowloaded ADE, activated the ADE, downloaded the same books, and did the same things I did on the Mac. The Nook doesn't show in the library.
After spending an hour on the phone with Nook tech support, the tech had me do two things that may help you.
1. Uninstall ADE and reinstall it. This didn't work for me, but it has worked for others.
2. Open your Nook drive on your computer and copy all books, etc. that you want to save/keep to a folder on your desktop. If you have downloaded books from Barnes and Noble, these will be in the "my B&N downloads" folder.
a. Remove the Nook USB cord from your computer.
b. On your Nook, go to settings and unregister your Nook (assuming you have WiFi at your location).
c. While in settings, set your Nook to factory settings. This will erase all your books, etc (that you have previously copied over to your desktop).
d. Register your nook again.
e. Attach your nook to the computer.
f. Open the ADE. Wait a few minutes. You should be able to see the Nook in your library.
g. Open the folder on your desktop in which you copied those items you wanted to save. Drag and drop them back into the "my B&N downloads" folder in your Nook drive. You will also see a new folder called "Digital Editions".
h. Drag and drop the books in your ADE over to the Nook in the library column. These will appear on your Nook under My Library in My Documents.