In order to get your free Steam key, you need to
- Login on http://www.dlh.net/ or register a new account.
- Visit Steam Keys page (Profile > Steam Keys on tab menu).
- Scroll down to the "A Story About My Uncle" giveaway, and press "Get your Steam Game here". Keep in mind that there are hourly/daily limits so if it's not working at the moment, you should try again later.
The Steam key will appear on the same page. Enjoy!