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Which sdk are you using to export?

what should the Make mdl be?You can use the P3D 1.4 SDK for exporting to P3D v2 mdl files. Aslo for Makemdl you specified the MCX path which is incorrect.
I suspect this might be a P3D model, which allows bigger triangle counts. In that case exporting to FSX might not work.
Which format you select in the export window?

fsxMakeMDL should point ot the MakeMDL.exe tool from the FS2004 SDK. You only need it if you will export FS2004 MDL files of course.
OK, so you are saving as FSX MDL, in that case the P3D 1.4 SDK is the right one. For which version was the model made that you try to convert?