Is it a nook eReader or a Nook tablet?
ADE will not generally recognize tablets, you must authorize the device from an app on the device, and copy books using a file manager.
ADE 2.0 fails to recognize some eReader devices, try 1.7.2.
There are lots of bugs in ADE2.0 (and 2.0.1). Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
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
Some people have found ADE trying to upgrade automatically.
It appears (not 100% sure) that if you install ADE2.0 as a new install (not as an upgrade) that your 1.7.2 will continue to run.
Probably best to say no if 2.0.x installation asks if you want to migrate your library.
eReader Nook HD
Does the Nook show up on your file manager (Windows Explorer, Mac Finder) as a mounted drive?
If not, the problem is before you even get near to ADE. Check the Nook hardware and your USB connection.
then try v1.7.2 as above.
No, I can see my Nook from my computer.
Try dragging the book (.epub or .pdf file, not the .acsm) from the computer to the Nook using your file manager,
and seeing if you can read it then.