If you have old GMAX files and you want to export them for use in modern games...
Needed Files: FS2004 SDK GMAX MSXML 4.0 Service Pack 3
First install the fs2004 sdk gmax setup (we only need the folder plugins). Copy all files from the sdk folder **plugins** to your main gmax .../gmax/plugins folder.
Install the MSXML Package.
Second open GMAX and export your model into the (Flightsim Aircraft Object (*.MDL)).
At least import the exported "*.MDL" model into "Modelxconverter" and export it at any format you needed (for example in the "*.3ds" format).