I too am using the Nook for Windows app with a .com email address (on Windows 11). I checked the help section in the app, and some of this may help if you need to get support:
Nook for Windows help email address:
nookforwindows@book.com
They also claim to offer support through Twitter:
@NOOK_Care
And this is the support page linked from the app, which may be out of date considering the landing spot is for Windows 8:
http://www.nook.com/support/faqs?cou...evice=Windows8
Only thing I noticed around registration limitations was needing to use a Microsoft account if you are outside the US and UK.