From the trainstation Arosa, you can reach the valley station of the Cable car Hörnli-Express with the local bus. Continue by foot or with the Cable car to Mt. Hörnli and further to Lake Urdensee.
Follow the signs from Chur to Arosa and the main road through the village till you reach the parking garage Innerarosa/Brüggli on your right.
Park your car in the parking garage Innerarosa/Brüggli or on one of the few parking spots at the valley station of the Cable car Hörnli-Express.