If the nook is a dedicated ereader, it should show up in Digital Editions when plugged into the PC,
and you copy books from there.
(If nonDRM books, use the superior and free Calibre software instead of ADE)
If a tablet nook ...
You need to install an ereader app such as Bluefire on the device, and then 'sideload' your ebooks from the computer,
that is copy them using a file manager such as Windows Explorer or Mac Finder onto the device which should show up as a mounted drive.
There is also an Overdrive app, which will allow you to borrow, download and read books on a device without using the PC at all.
Some people have problems registering Overdrive app with their Adobe ID.