The Sphere on COG create a little sphere part with name CenterOfGravity on the centre of gravity.
Operativity
to add The Sphere on COG on single element, drop the tool on the part (in this case, will be create an Intellishape named CenterOfGravity into the part).
to add The Sphere on COG on the assembly, first select the assembly, then drop the tool into the scene.