Activating gifted copy of the game on Steam

[Curvin]

As per title, some time ago I was gifted a copy of this game by a friend, however I never got around to "activating it" on Steam and I do not know how, I mean I can still play the game alright, but I would like to activate it on Steam in order to grab the Oceania DLC currently on discount over on Steam.
Thank you.
EDIT: Things have become more hectic than I expected, I figured out how I would normally go around to do what I asked about, but the product is already "activated", I cannot register the serial number on Matrix nor Steam because it's already been redeemed, and now I have no idea what to do with this copy or how to buy the DLC for it, help?

[Vic]

Sorry for the late reply, but you should take this up with Matrix customer support (the publisher) not with the developer (me). Good luck!