I am having the exact same issue. Called B&N customer service, their troubleshooting didn't help.
HP EliteBook, Windows 7, ADE 4.0 (downloaded today), Nook Simple Touch
For some reason this isn't working for me So aggravating!!
Here's the skinny if you (after downloading and installing V.3 or 2 and still not working, you ahve to delete the 'Digital Editions folders from your nook folders to officially 'unauthorize' your device. So incredibly confusing, but incredibly happy to have finally figured this ish out. HAPPY READING!!
1. Plug the NOOK on the computer using the USB cable.
2. Open “My Computer”.
3. Open the “NOOK” drive.
4. Delete folders named “Adobe Digital Editions” and “Digital Editions”.
5. Go back to “My Computer”.
6. Right click on NOOK drive and select Eject.
7. Unplug the NOOK from the computer.
8. Open “Adobe Digital Editions” application.
9. Plug the NOOK on the computer.
10. Pop-up window will be appearing saying “Device Setup Assistance”.
11. Select “Authorize Device” to authorize the NOOK.
12. You will see the NOOK folder under Bookshelves column.
13. Drag and drop the eBook on the NOOK folder.
14. Eject and unplug the NOOK.
15. Using the NOOK, go to “Library”.
16. Open your eBook and enjoy reading.
Sep 2016 - I have only recently bought a Nook (Simple Touch Glowlight 4GB) and have had the error, 'User not activated', with 3 of the eBooks I have bought from online retailers. (I live in the UK and we are not allowed to buy directly from Barnes and Noble !!)
I have been receiving an '.acsm' file then downloading the EPUB file using Adobe Digital Editions. At this point I can read the book on screen on my iMac but when I sideload it on to my Nook, the Nook does not allow me to read it - 'User not activated'.
I have then used an independent, free, eBook program called, 'Calibre', to load the first, downloaded EPUB file into Calibre's own 'library'. At this point I go to the Calibre Library folder on the Mac and copy that version directly on to the Nook.
I don't understand what the difference is or why it works - I did wonder if Calibre stripped out the copyright (DRM) stuff from the eBook but I don't believe it can do that.
I hope this helps.