hemax_material should be a detial attribute that uses the following syntax for specifying a material:
<PathToMatFile>:<MaterialName>
For example if you had a material library with the name RockMaterials.mat and you wanted to use a material called RockInterior, the attribute should look like this:
C:\materials\RockMaterials.mat:RockInterior
The hemax_matid should be a primitive attribute that specifies the material ID for each face of the mesh.