Go to your Steam library, right click on FINAL FANTASY XIV: A Realm Reborn, and select View CD key.
Log into the Mog Station, select Your Account, and enter the keys under the available options:
Enter the Pre-purchase Bonus Content product code using the Enter Pre-order Bonus Code (Early Access Code) button.
Enter the Standard/Collector's Edition product code using the Enter the Registration Code for the Expansion button.