I'm so sorry to hear about this! I'm on the PS3 and had the reg code-registered-to-the-wrong-service-account problem and it took me a couple days of sifting through forum posts to find contact information.

Here you go:

310.846.0345
NA customer service; this is a number out of Inglewood, CA so mind any long distance charges. When I tried calling it took HOURS to get through (2.5 hours +) and most of the time I got a busy signal when I called. Just keep trying and try calling RIGHT when they open up, which is 9am PST.

Chat support:
support.na.square-enix.com/faqarticle.php?kid=67428&id=496&la=1&SQEXSC=12c3t5gpkt1l8vpu7ou3jo0a66

Scroll to the bottom and click "Additional Assistance" and choose your options from there. Again, I would recommend getting in RIGHT as they open. I was able to get through on the chat faster than the phone at one point but you may want to go for both and see who picks up first, since you may get booted from the chat queue.

This link also works for EU players, just change the NA to EU in the address.

Good luck!