The MTL files should be right beside the OBJs.  At some point, you import the OBJs into Poser;  at that time, Poser will read the OBJ's associated MTL to determine the material zones and UV mapping.  Assuming that you're going to save the model as a prop once you get the textures applied, Poser will no longer need the OBJ/MTL files, so you don't need to have those in your runtime.  You can import them right from the desktop folder.  But the texturing images (JPG, TGA) should go into runtime>textures>Complete Cave. 

     To be clear, this is distinct from Poser props and figures which reference remote geometries (the geometry has been stripped out and is stored somewhere in runtime>geometries).  Those remotely referenced geometries must be left where they are or all models which use them won't be able to find them.

     What you will be doing is importing geometry meshes, texturing them, and then saving them as props in your props library.  Poser will save them with the geometry embedded within the prop file.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5