1) Search on the forum and check for the android file transfer progam.
Actually, here's the link - free download
2) Go to Settings > Storage Management and tap on USB Connectivity. Then choose PTP (Camera) mode.
Then plug into your Mac.
You can also go this Top 10 Nook HD+ Issues thread and look at the fourth post.
I had the same problem with my powerbook. After going around in circles for nearly a week with several B&N representatives and managers I was finally told by phone (customer service) that the new nooks are NOT compatible with the older macs like the nook color and tablet were. I was told I would need a macbook. Thankfully I have an old p.c to hook it up to because I can't afford a MacBook. I really wish they would have had this listed somewhere in their specs or advertising. It was very frustrating for me as I'm sure it is for you. Sorry, I wish I had a better answer.
EDIT- I didn't realize you said you own a MacBook. You shouldn't have any issues with hooking your Nook up. Hopefully my response will at least help other people who are searching for an answer as to why they can't get it to work on the 'dinosaurs', lol