The ability to do detect the center of gravity of a machine automatically is not availalble in SimMechanics 3.2 (R2010a_pre). You can use the following approach as a workaround:
You can define all bodies with their CG defined in the World Reference plain and then write a MATLAB script to manually perform this computation. This would use the FIND_SYSTEM and GET_PARAM commands to query this information as such;
machineCG = [0 0 0];
for i = 1:length(h)
machineCG = machineCG + masses(i)*CGs(i,:);
machineCG = machineCG/sum(masses);
In the case where all of the body CGs are not necessarily defined in world coordinates you may use the Body Sensor block to do this conversion for you but this requires hooking this block up manually to every body block in your model. Connect the body selector to the CG of the body block and select World Coordinate system for the output of position and send this information to workspace or wherever you wish to compute the CG.