(copy-pasted from a facebook message I sent my friend who couldn't find his code)
go to your email and search your inbox for "square enix"
look from a email from webhelp.v4
the subject should say
square-enix inc. - Order Confirmation (Order#XXXXXXXXXX)
click on the link that says findmyorder.com
ctrl cop and paste the order number from the email and type in the password for the square enix account you used to buy the game
FINAL FANTASY XIV: A Realm Reborn Pre-Order Bonus Package Final_Fantasy_XIV_Early_Access_Instructions.pdf Serial Number: XXXXX-###-XXXXX-### Electronic Download
youll see something like that

the serial number for the game and preorder bonus package is also listed at the very bottom of the email. noticed this a few minutes later
when you go to na.preorder.blahblahblah, and type in the serial number for your preorder bonus package, it will give you a different 20 digit code that you need to type in the first time you log in to the game on saturday morning.
WRITE THIS CODE DOWN IN CASE THEY DON'T EMAIL IT TO YOU