Is it at least possible for you to click START, or the click problem starts with the gameplay?
About the gameplay: once you click START you can do a basic attack with left click that deals low damage, with it you can attack enemies or destroy objects around the scene. Those objects leave scrap that you can grab with the right click, it start orbiting around you and deal damage based in the speed you make them rotate.
And just in case you make the click work somehow: we'd like to mention there is a bug in the actual build that prevents you dealing damage to the enemies that don't spawn at the center.