Adobe creates a small ID file for each ebook that's downloaded. Part of
that file is the Adobe ID of the user who did the download. Thus, the
ebook is linked to the user who downloaded it. The message is telling you
that you're not that user and you cannot access the downloaded ebook unless
you are. That's the way it is - because the Digital Millenium Copyright
Act of 2000 says it should be. Sorry!
ADE does not (yet) support content purchased from Barnes and Noble.
You have a few choices: