The following question comes up way to many times:  How do I convert a mdl file to a format that GMax can read? (Applies to FSDS,3Ds, and any other design software)

The answer: You can't! The author won't be happy and you're not allowed to. Second, you just can't, period! You'd also be better off emailing the developer for the source files, or even modeling the aircraft from scratch.

Happy modeling!