If the book is not getting DeDRMed you will need to go to alf's website to get help. MobileRead has a policy not to give DeDRM instructions, guides, or help (although we can suggest "google apprentice alf").
As far as actually registering Kindle4PC, have you tried logging into your Japanese account using a proxy to pretend your computer is actually in Japan?
http://www.google.com/#q=windows+http_proxy