Why can't I access downloaded library books?

I just installed and activated Adobe Digital Editions and Adobe Reader. Then I downloaded several e-books from the public library; these books are visible in my downloads on my computer--but Adobe doesn't recognize them nor does it recognize my Nook. When I try to access the downloaded books, I receive the message: Error! Check Activation.  I have restarted my computer and done everything I can think of. Thanks in advance for your help!