I've rooted, installed GAPPS and so on many Nook HD+ tablets; most for my friends, since they have no idea how to do it. I've noticed this problem occurs due to the Nook not being installed correctly in ADB mode.
What I would recommend is going into
Device Manager, find "Android Composite ADB Interface" and make sure it has a green-chip-like icon on the bottom left corner of the icon. If not, then it's not functional. To solve this, once your tablet is connected, it should be detected as a media player/portable device, with the name BNTV600 (I think it is/was - all this is still in Device Manager by the way). Right click on the device, and attempt to change the portable media player into a "USB Composite" device. That should solve the trick.
If you're still having problems, I can connect to you via TeamViewer remotely to access your computer, whilst your tablet is connected and solve the problem for you (if I can). If you'd like, I can completely root your tablet for you. I've done it for a couple of friends over TeamViewer, so if you'd like, I can do it for you too.
Best of luck.
P.S. Try disabling "USB Debugging" mode and set "Enable ADB".
