Is it a dedicated Nook eReader, or a Nook tablet?
For a tablet you some sometimes need to 'sideload' the .epub file; that is to copy it from the PC to the Nook using a file manager such as Windows Explorer. As long as[a] you have Adobe DRM compatible ereader software installed on the Nook and [b] that is registered with the same Adobe ID as your PC, you should then be able to read the file on the Nook.
Bluefire is one such app I have found good. There may well be one preinstalled on any Nook tablet (???).
For a dedicated eReader, you should also be able to sideload to the reader the same way; but might have problems getting it registered to your Adobe ID without ADE being able to see it. My dedicated Sony eReader needs Sony software installed on the PC before ADE will see it. Installing the Sony Reader software also installs a driver; and that driver enables ADE to 'see' the Sony even if you never run the Sony Reader software itself. It shows up as a drive without the driver software, but not as an eReader device. Maybe there is some similar stupidity with Nook eReaders?