I had this problem earlier. I play on PC and purchased my copy straight from Square. I redeemed my code to get the mog station code. I was originally clicking on the giant "Enter the registration code for the expansion" option and inputting my code on the new page and kept getting invalid code.

What worked for me is that, beneath that giant link you can click on, there is a smaller link/tab for pre release. Click on that and input your code. It didn't work for me the first time, until I switched the code to all capitalize letters, then I was able to install.

I hope this helps someone!