The top, inertia, field of the mass component is where you set the size, or mass, of the mass. The SI units are simply kg, but the english must be lbm. Get this by lbm=lbf/386.5 where lbf is the mass's weight in pounds.
The initial condition, both horizontal & vertical is where you set the state of the mass at the beginning of the simulation. If it is to be pulled up, then set free to bounce, set the displacement & so forth.
