Okay I'm being lazy now after searching for a half-hour on how to use a converted steam key. I logged into Steam to find that the only result for "planetary annihilation" is for $90. I'm not sure if you can use the key after you go through a few steps, but don't know. what the hell is the deal..
Once you created your Steam key via your own Uber account, you go to Steam, go to library in steam, click "add a game" select "activate a product on steam" enter your new steam key... let steam download the game, play and enjoy.