Don't know much about the CenterOfMass prop, it's not something I have investigated untill your question. As I understand it (and I may be wrong), Auto Balance needs to be turned on in the Figure menu first, then you can select the CenterOfMass prop from the Props menu and move it via the translate dials. The CenterOfMass prop will show as two small concentric circles in wire frame display style (you can change the display stile if you wish).

When you turn on Auto Balance another prop will display as a green sphere, it is the GoalCenterOfMass prop, under normal circumstances it does not show in the props menu, neither can it be selected or moved. when you turn on Auto Balance the CenterOfMass and GoalCenterOfMass prop may be superimposed.