You need to CHECK the trimesh settings for every mesh (part of the body etc.) you export. If you dont want something to be rendered but you need it ingame nevertheless uncheck box "render", thats render = 0! That way you use bones. The way is to use shadow on and render off because skinmeshes dont cast shadows at all and dont even accept any king of trimesh settings above the skin modifier.
And Gmax is the old 3dsmax 3 or 4 without the tools advanced modelers (rendering, lights camera etc.) use so what works with gmax technically should work very similar in 3dsmax!
And I suggest you take a look at the original minotaure model from bioware and see how everything works. How it works in 3dsmax and ingame.
Trimesh settings are what the aurora engine can read so to say. The engine dont understand just a mdl file from 3dsmax and thats it. Thats not how it works. you have to tell the engine what to do with the model '>