[Nook HD, HD+ Q&A, Help & Troubleshooting] HiddenG:
No problem at all, and I do apologise for the inconvenience being caused to you, as a result of not being able to register your device. I'll do all I can to assist. ;)

After you install CyanogenMod, you can always return to the stock ROM; by formatting/erasing everything on your Nook and installing the default/Stock ROM back on (don't worry, it's easy and fun). If you want, you can try installing the stock ROM provided by leapinlar (another developer/great contributor here at XDA) over your current setup, and perhaps you maybe able to register you device then. I flashed the stock ROM provided by leapinlar for a friend, and he was able to register with a new Nook account (although he did own the Nook from new). Or, perhaps you could try this:

Turn off your Nook;

Turn on your Nook;

Immediately

after turning on, and you see the text "Nook", hold the power button until the device turns off;

Do the above steps eight (8) times.

Make sure you keep a note of how many times you have done the above, to ensure you have actually turned off the Nook 8 times. What doing the above will do is, it'll reset your Nook. After that, you can try registering it then and hopefully it should work. Sorry I didn't mention this earlier, it didn't occur to me. If it doesn't work, you can try installing CyanogenMod (CM) or try installing the Stock ROM again.

You can install CyanogenMod, which is basically a full Android 4.2.2 setup (later versions of Android are currently being tested/developed for the Nook). With CyanogenMod you do not need to register with Nook, and you can begin using your Nook after installation. However with CyanogenMod, you will need to register with Google and optionally with CyanogenMod. Once you register with Google, you can access all Google services such as Google Play (the app store). The optional CyanogenMod registration allows you to monitor your devices and such.

If you'd like to install CyanogenMod, or try reinstalling the stock ROM, you must have an SD card. The SD card must be blank, and you must be able to connect the SD card to your computer, so you can write to it directly. I would recommend a minimum of 4GB SD card capacity, but if you have anything else laying around, I guess we can make do with it. The absolute minimum you require to install CyanogenMod/Stock ROM is 2GB.

If you'd like to install CyanogenMod, please see this guide: http://xdaforums.com/showthread.php?t=2317500

When downloading the CyanogenMod file, ensure you download this version: http://download.cyanogenmod.com/?device=ovation&type=stable which is a stable version, ready for everyday use. ;)

After following the guide I have just linked to above, you should have CyanogenMod running on your Nook perfectly fine. If you have any problems, please let me know, or post in the above thread.

Before you try CyanogenMod, if you would like to try "fixing" the stock ROM (i.e. see if you can register) by installing the stock ROM again, overwriting your current setup, please reply here and I will detail how to reinstall the stock ROM and reset it, which should hopefully allow you to register the device.

All the best. :)