Connect your Nook to your PC via the USB cable.
Follow the prompts to authorize your Nook using your Adobe Digital Editions ID (the username and password you created.) [You only need to do this step once.] If you have difficulty, click on the "Authorizing Problems?" tab (to the left).
An icon of your device will appear on the left of the screen, next to "Bookshelves." Drag and drop the book from the right side of the screen onto the Nook icon to transfer the book.