I'm having the same problem but UK one, I've checked email/password over and over with no luck. I had my wii and dsi linked fine, to test I unlinked my wii and now trying to link gives the error, not tried the dsi as its not updated to latest fw(on 1.43e) due to using m3i zero and don't want to get it blocked.
I've emailed them today and have tried phoning 3/4 times but getting the lines are busy message, will see if/when they reply to the email.
bought the white 3ds xl with mario kart pre-installed(well on the sd card) bundle yesterday, do you have to have the nintendo club account linked to the eshop to be able to benefit from the free game or does it work with out? I was wary to register the 3ds xl in case it was the case but ended up registering it just a little earlier now to wait for the code for mario 3d land and hope it works fine.