General tips:
If you messed up and bought the pc codes instead of steam codes, you'll have to get them switched to steam codes. (Which means getting hold of Squeenix's tech support) Also there are two codes, one for early access/pre-order bonus and one for the expansion itself.
When entering the codes make sure you use caps, lower case doesn't work. If you copy/paste your code make sure no blank spaces get copied into the code.
Not sure if Steam code is the same as pc code but the email I got gave a code that I had to enter here: https://redeem.finalfantasyxiv.com/na in order to get the actual codes to put in the mogstation.
I hope this helps, if not....calling Squeenix may be the answer if you can ever get through.