Can't Purchase unlockables

[wonderkid360]

So, I wanted to buy this extra challenge that's available, but when I click on Store it says: In Order to access The In-game store you must 'Enable Steam Community in-game' via Steam Settings.I've looked but I can't seem to find the Steam Settings.
All help is greatly appreciated.

[mattdamon1990]

could try going in game and pressing shift-tab. Brings up steam community in game.