Currently it's (still) not possible to register devices bought in different regions, into one account: http://forums.dlink.com/index.php?topic=52632.0
On the bottom of the post it's mentioned that "D-Link is in the process of changing this regional-based design in favor of a single global mydlink account that has no geographic dependencies". That was February 2013. It's now July 2014, is there any news?
I have bought hardware from all over the globe from different brands and I have never seen something silly like what D-Link has here.