In order to get your free Steam key, you need to
- Login at http://www.dlh.net
- Visit Steam Keys page (Profile > Steam Keys on tab menu)
- Scroll down to Serial Cleaner giveaway, fulfil all requirements and get your Steam key. There are hourly/daily limits of available keys so if it's not working at the moment, you should try again later
The Steam key will appear on the same page. Enjoy!