If you bought the base game from Steam you have to buy all current and upcoming expansions through Steam. If you have a regular non-steam account then try buying it from your account management page in the mog station. After logging in there you should see a chart with all your registered content and the ones you have yet to buy, simply click "Buy" on the one you want. If for some reason that still doesn't work, you can buy a key from any of the several authorized resellers like Humble Bundle store or greenmangaming.com. Please remember that a Steam key will not work on a non-steam account and vice-versa. I know I'm repeating myself with that but it's a crucial piece of info and is more often than not what is causing problems for people.