Most of the recent Kickstarter updates describe
how to claim a D:OS 2 key.
Basically, you need to create an account on the
Larian Vault website, if you do not have one already, and link the Kickstarter/PayPal pledge.
After doing that, the tier details will be listed in the Vault, after clicking the D:OS 2 Kickstarter menu entry. Under 'Your Choices & Input', scroll down to the game listing(s) and click on the OK button to request a Steam key (currently that is the only option; GOG keys will be added 'soon').
After a conformation, you'll receive a Steam key, which you can copy and paste into the Steam client (in the Game menu, select 'Activate a Product on Steam...', then Next and I Agree to enter the key).