Note: You can only show the center of gravity when the Calculations dialog is open.  This avoids unnecessary computations.

Select one or more objects.

Click Calculations ( ) in the toolbar.

The Calculations dialog opens with the CoG tab active. (For more information on the other tabs, see Calculating Distances and Calculating the Center of a Sphere.)

In the CoG tab, you can see the coordinates of the center of gravity with respect to the world frame are given as well as the total mass of the selected objects.

Select one or more objects.

In the CoG tab of the Calculations dialog, select the Show check box.

The center of gravity indicator is shown:

You can make it easier to see the CoG indicator using the Render on Top and Indicator Size options. Render on top controls whether the CoG indicator appears on top of the model so it is visible even if occluded. The indicator size can be used to change the size of the CoG indicator.

The center of gravity indicator as rendered on top:

Select one or more objects.

Click Create Coordinate at CoG.

The coordinate is placed. In the Tree View, coordinates are indicated by this icon:  

The orientation of the coordinate is the orientation specified by the selected body, or the first selected body (if multiple bodies are selected).