Adobe Digital Edition software for Nook ebooks

Hi:

I have just downloaded Adobe Digital Edition software on my iMac in order to copy ebooks from the library onto the Nook device.  However I signed up "anonymously", turns out now I can't copy ebook from my computer onto Nook because I should have signed up with a password.  Can you please help me??  What do I need to do for my iMac to recognize connected Nook and copy a borrowed book from comp onto Nook?

Thank you.