Can haz questions?

[Dogempire]

If that's the new one from the Mog Station. Click on "Your Account", then hit "Enter Item Code". That should let you redeem the mount.

I already redeemed it, the game gave me a white devil identification key.

I don't really know how to use it.

[AskaRay]

I already redeemed it, the game gave me a white devil identification key.

I don't really know how to use it.

OOOOH
Oh okay I understand now...sorry X)
When you reach, I think, Main Story Quest level 20, you'll be able to rent/own a chocobo, which is your first mount (it's when you choose a Grand Company to ally with). That's when you can start using mounts. When that's available, just "Use" the identification key and the mount will be put into your mounts you can choose from.

[Elleia]

I already redeemed it, the game gave me a white devil identification key.

I don't really know how to use it.

If you have the key in your inventory, just right click the item and select use. The mount will be added to your mount list automatically.