That car/folder Hammer_PC you shall have in this folder GameData\Vehicles\PaceCar
in the rfm file add SafetyCar = Hammer_PC.veh
that wec rfm shall load hammer maybe you have many other mods installed in same rfactor folder ?
when u start the game klick on the mod you wanna play first
remember that when close rfactor take and load the mod before close rfactor that you wanna start with when u start rfactor next time
if u have a Aston Martin GT3 as pace car with tha rfm file that wec comes with it seems to collide with another mod you have installed