Matrix,

First, in gmax, position the model so that the grid crosshairs are at the center of gravity.  This is typically at or near the waterline, the engine thrustline (average for multi-engine aircraft), the Center of lift, or at MAC (typically 25% back from the leading edge of the wing).

Here are some shots of the Commander calibration box setup that show the grid and aircraft placement.







Once that is done, you can use the Tape in gmax to get your measurements needed for the aircraft.cfg file.  Look under the Helpers Icon over the Rollout Stack.

If you need more explanation, please ask.

[EDIT]  I just found this visual tut from Firestriker's site I believe that may help.
http://www.oregon-coast.net/Tutorials/CofG/index.htm