Copying a reply from another place. Might help:
(source:
http://bookclubs.barnesandnoble.com/...ok/td-p/477693 )
I see no mention of Adobe Digital Editions anywhere in this litany. You need to download the book to ADE first, then copy it to your nook. In addition, your nook needs to be an authorized device in ADE. Are you sideloading the book itself or the .acsm file? the .acsm file is simply a link back to the library to securely download the book.
the sideloading into "my documents" is not correct - it was a workaround in 1.1.0 because ADE was not connecting correctly. What you should be doing is dragging and dropping the book in ADE to the nook icon on the left side of ADE (where it will appear if it has been authorized). This will place the book in the "Digital Editions" folder on your nook, which is the correct place for it to appear.
If you need more info, the New York Public Library actually provides video tutorials of how this is supposed to work. Even if they're not your library, you can access the videos and get more info.