You download and install Digital Editions (ADE) on a Windows or Mac computer,
and then transfer books from that to the Nook.
You can download ADE from http://www.adobe.com/uk/products/digital-editions/download.html
There are lots of buts in the latest version, so also consider ...
There are lots of bugs in ADE2.0 (and 2.0.1). Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172