Is this a vanilla model? Strange I've never encountered one that does not open in MDLVIS.
Does it open with 3dsmax m2 importer?
What I usually do when a model cannot be opened is that I export it to OBJ/3DS and then use the 3ds/Obj > MDX converter and then run that MDX through the MDLXConverter so that you get an .mdl file. Then copy-paste the geoset information of that .mdl to the geoset section of nullmodel.mdl, which can be downloaded in one of my old tutorials