A quick Google of "nook for pc epub" found these instructions from HowTo Geek:
http://www.howtogeek.com/howto/26332/read-all-your-ebooks-on-nook-for-pc-and-portable-devices/
...basically, in nook for PC click "my library / my stuff," click the "Add New Item" button, navigate to your file and click "Open."
Also try the free app Calibre (http://calibre-ebook.com/), which will connect to most eReader devices, and can automatically convert ebooks and other econtent as needed on-the-fly without you having to think about it. If you like it please consider a small donation, so their programming team can buy lunch occasionally.
