No no no! I am not writing about the sourcecode of Mario Kart. Source is the graphic engine created by Valve is used by Halflife 2 for example.
So Mario Kart Source is a modification for Halflife 2 which tries creates the well known Mario Kart feeling on your PC.
Apart from some pictures and videos there is nothing to download yet. But if you want to give the developers a helping hand, you can contact them on the Mario Kart Source Website.