Precisely what is the URL of the page you're referring to as the 'main website', SE have several regional ones linked off their global main http://www.square-enix.com/ and they also have several other 'main' sites for various aspects of their games, the main one for FFXIV being na.finalfantasyxiv.com for the NA region for example.

At the moment I for one can't think of which site you're trying to enter a registration code, apart from the Mog Station which is where I presume you set up your subscription: https://secure.square-enix.com/accou...svc/mogstation.