You can register your nook without US-IP address using the WiFi connection. There isn't any IP validation while registering. There are 3 possible reasons:
1.) mail address and/or password are wrong
2.) B&N servers are down
3.) The date settings on your nook are totally wrong, resulting in an error while establishing a secure connection to the B&N servers. The nook doesn't accept the certificate issued by the server, cause your nook is somewhere in year 2000, but the certificate is valid from 2009 to ...
The nook uses the cellular network, to get the current date & time. Since you are outside the US, your nook can't connect to the cellular network. B&N modified the firmware with v1.2 to get date & time over WiFi using a NTP daemon. Make sure your nook uses the newest firmware. And make sure your router allows port 123/UDP to pass through. If this doesn't work, you can softroot your nook, to set the date manually:
http://nookdevs.com/Tips_and_tricks#International_Tips