The first step is to create an account if you do not already have one. You can do that here:
https://samelogin.slitherine.com/Account/Register
Then you need to login here:
https://www.slitherine.com/login
Once logged in, click "MY PAGE" and you'll see a list of games you own. If the game is not appearing on this list it means it has not yet been registered. To register your game click the "REGISTER SERIAL" link (4), and follow the instructions to register your key.

Once your game is registered, find it on the list and click on "FILE DOWNLOADS" (1).
Click the Download button for downloading the installer (2), the update (3) or any other file.

(*) Download server location.
(**) Number of users downloading the file / Max number of users permitted by the server