In order to get your free Steam key, you need to
- Visit Game Giveaway of the Day and press "Mighty Party: Back to Transylvania" button
- Post about this promotion on Facebook. If you don't want to publish it, press "Alternatively you can get a personal download link via email" and insert your e-mail adress
- Press the download button and log-in through Steam.
The Steam key will be send to e-mail used earlier. Enjoy! :)