http://www.freeflightdesign.com/tutorials.htmlThere are a couple of gmax VC tutorials there.
In it's simplest form, your basic "shell" for the VC is you exterior cockpit fuselage, cloned and all polygons flipped.
You will make two small boxes (0.1units cube) Name one exterior, and all your exterior parts will be children of this node. Name the other one "interior" and all the interior (i.e. VC) parts will be children of this node.
Typically, you may want to create the different models as separate scenes, before doing a final merge of the scenes.