https://www.slitherine.com/login
Once you are logged click "MY PAGE" (1) link.
The menu on the right lets you navigate the Member's Area and register serials. Either browse to find the game you want to register and if you cannot find your game registered click the "REGISTER SERIAL" link (2), and follow the instructions to register your key.
Once you have registered your serial you can select “Get Steam Key” (3) for any game which is available on Steam and this will redeem a Steam key.

Now go to Steam. In the Steam client select “Game” from the top menu and “Activate a Product on Steam” and enter your key.

If you have any questions just let us know.
The Slitherine Team