Sorry if this is the wrong section.

I'm wanting to initiate Heavensward DLC that I bought off Steam.
I was wondering how this is done? The download was instantaneous has given me a key.

When I try this key on the Mogstation it doesn't accept the code? Am I doing it right?

Other information would include me only recently getting to 50 Dragoon and not completed the main storyline? But I have competed it up until the filler missions.