Well, in a nutshell, it's like this,,,
You need 3dmax or Blender (or another 3d editor as long as it can save to .obj, .dae or .3ds) and 3dSimEd.
You build your car, Here is how to build a car,
http://www.3dtotal.com/team/Tutorials/fiat500/fiat500_m01.php
Then you uvmap the car and export wire frame's so you can make the textures.
Then if you have 3dmax you set up your materials and scene data and export it to gmt.
or if another 3d editor you export to obj, dae or 3ds and import that to SimEd, set up your materials and export it to gmt.
It's best to look for isi tools version 1, in the zip is an example car max file, it will show you how to set up the materials.
Then you edit the data files, you need to make a gen file for the spinner and the track and you need various other files for physics. But physics isn't my thing so I can't help you there.
Did any of that make sense?