When I hook up the exporter's Engine Path to Left 4 Dead 2's bin folder (SteamApps\common\Left 4 Dead 2\bin\) and export a DMX, L4D2's studiomdl will not recognize the DMX format.

Left 4 Dead 2 recognizes DMX version Binary 3 Model 15. When the exporter is set to L4D2's bin folder it will export in Model 18, which L4D2's version of Source predates. Model 18 was introduced with Alien Swarm.

Hope this helps. Best wishes.