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.