As an aside, this is one of the reasons you guys are so great. Fan comes out of the woodwork years after the kickstarter, reports missing rewards, and within an couple of hours, the Managing Director comes in to help resolve it.
To play your new games, just launch the Steam application on your computer and log in using your Steam account name: syntheticdawn. Your games will be available in Steam on the "Library" tab.
Any Downloadable Content (DLC) you've purchased is automatically downloaded and will be accessible from within the base game on Steam. To view your new DLC from within your Library, right-click on the base game and select "View Downloadable Content."