Sorry if I'm wrong here (it was a while since I did this!), but if I remember correctly you can just go back to the preorder site, put in your preorder code again and it'll give you the code needed for the mogstation (it'll be the same code no matter how many times you do it).

I think there were problems though if you had a preorder code from NA and an account that's EU (or vice versa) as the code it gave wouldn't work in the other sites, not sure if this is actually relevant though, just mentioning.

Sorry if I've been no help, but just trying the pre-order site again should give you the code you need.