You don't do anything. EVERY Copy that is played right now is a Early-Adopter Copy. There is no way that you can play the game right now and not get the Bonus (well technically pirating the game is but... i will just assume you have a legit copy in your Steam Library).
Everyone who owns the game right now will get the Early Adopter Bonus. And no it is not specifically marked as that in the Steam Library. It doesn't read Adopter Edition" or anything. But you WILL get it way.