You get the Nook app from the App Store. Just search for Nook. Be sure to get the iPad version.
Once you have the Nook app on the iPad, you open it. Somewhere, either in the Settings app > Nook, or in the Nook app itself (I haven't used the app in a long time) you will enter your Barnes and Noble ID and password. Once you do that you will see the books you've purchased on the Barnes and Noble site, and can download them to the iPad. You have to have an internet connection, of course.