an easy way to put your model in FS is to simply copy the folder of some other plane, then replace the MDL flie with that of your plane/spaceship

(you need to first export the model from Gmx into MDL format, that's what MakeMDL is for)

what happens next is that in FS, your plane will behave like the plane you switched it with

your aircraft may not look right, it might hover or appear half-burried in the ground when landed

to correct that you have to edit the contact points (this is done outside of Gmax)...
there are a few good tutorials on how to do that and other stuff out there, be sure to look for them

hope it helps

Moach

I have tried the method you had suggested, and it doesn't work.  The aircraft doesn't show up in the program.