If you're referring to the North American Square Enix Online store, I believe the early access code was written in your first confirmation email.

The format of the code is AAAAAA-123-AAAAAA-123 and can be redeemed on our Pre-Order Code Redemption website: http://preorder.na.finalfantasyxiv.com!

Redeeming it there will give you further instructions on your next step!