Assuming you're an NA player:

The number for customer support is 310-846-0345 (unless this is outdated, it's just what I have written down).

If you're uncomfortable calling, there are various ways to access the form online for contacting support, buuut...

https://support.na.square-enix.com/c...p?id=5382&la=1

Contact Category: Accounts/ID
Sub Category: Registration Code

Contact details: Write something about PS3 registration code problem, then click next. If it doesn't show anything there that would help with your problem, just click next again to proceed to the login page. Log in and follow the instructions from there to submit a ticket.

It's a little confusing at first, so just read everything carefully.