Just follow these steps to install the game and play for free until early Monday morning...
1) Load up your steam client (or restart it if you already had it running)
2) Click on the "My games" tab
3) Scroll down to the "Not installed" section
4) Right click "Zero Gear"
5) On the popup menu click "Install"
See the first comment reply for an image summarising these steps.
Multiplayer kart combat at its best is now available on Steam. Nimblebit is celebrating their release of Zero Gear by extending the 25% discount and allowing all users to play for FREE until Sunday! [early Monday morning UK time]