If you make a pre-order, you will receive a pre-order code (either via email or on your reseller website. The time it takes for you to get it may vary depending on where you buy it).
This code will be shaped like this: XXXXXX-000-XXXXXX-000. This code cannot be directly redeemed on the Mog Station.
In order to get a proper code to activate on your Mog Station account, you have to "convert" that code into another here:
https://redeem.finalfantasyxiv.com
Make sure to log in, enter your code, and then select the proper platform you want for your code.
Note for Steam users: Others have reported that this step is not necessary, as Steam seems to provide directly the code that can be redeemed on the Mog Station. Please be aware that this might not apply to people buying a Steam key from another reseller.
You will then be provided a code (an email will be sent to you as backup) that can be redeemed on the Mog Station under the first menu entry on the top horizontal menu. There should be a button to register the early access code.
http://forum.square-enix.com/ffxiv/t...eming-codes%29