Did you actually register your game to your account? Before you automatically say yes, read the rest.
This shit is confusing, and stupid, but this is all the things you need to have done.
1. Register your pre-order code. You did this and got your early activation code which allowed you to play
2. Register your early activation code. This gave you access to the game from saturday through last night. early access ended last night.
3. Register your actual game code. This seems to be causing people issues because they think they registered their game, but they didn't. You should have gotten an e-mail from whatever retailer you bought the game from with a new code for this. Go to
https://secure.square-enix.com/account/app/svc/Login?cont=account&request=mogstation and log in, and click on Add Platform (icon looks like story quest line icon). From there you can add the game registration code, which gives the 30 free days and enables access. This is also where you will put in a payment method.
Using the mog station, you can screw shit up by doing the wrong option. I pointed out the add platform button here:
http://i.imgur.com/OW7n4qW.png
It's not confusing or stupid...
It's necessary for the game to succeed and if you read comments/announcements from Square, you would have zero issues following instructions...