To activate your copy of Short Oval Extreme if you have an activation code you need to download Steam and create an account if you don’t already have one, it’s free.
https://store.steampowered.com/about/
Once you have an Steam account created you can activate your code by opening the Steam app and select Games from the top menu, then ‘Activate a Product On Steam’
Once this is done you should see the game in your library, you can download and play it from there.