As the nook mounts on your Mac's Desktop as an external hard drive, you just have to drop the ePubs (PDFs, MP4s) into the correct folders on the nook.
A few things to remember:
1. When you plug the nook into the Mac, you must turn on the nook and slide the on-screen unlock in order for the mounting to take place.
2. A message pops up on the nook screen reminding you to unmount the nook's drive from your Desktop before you disconnect. Remember to do so. Once it's unmounted, the nook screen will change and it's now okay to disconnect the nook
3. Your Mac will drop "dot" files onto the nook which you won't see in the Finder; however, Pathfinder (a Finder alternative) lets you see invisibles and delete them.
4. If you want to encode videos for your nook, use Handbrake and the "iPod Touch/iPhone" preset. It's okay, however, to increase the frame size up to 854 pixels wide x 600 tall (obviously, keep the proportion correct). This preset leaves out "B" frames and it works like a charm.
While some folks really like to have an app manage their eBook library (like Calibre), I have a folder on my Mac full of my eBooks. Books that have been transferred to the Nook I set to yellow. When I have finished reading one on my Nook, I set its color (on my Mac) to red to indicate it's been read and then toss it off the Nook the next time I connect it.
Lots of different ways to work with this "half-an-iPad".
