mat files have a combination of textures in them, most in .dds format, some in 32bit targas, milkshape can use both of these file types, but you must first extract them from the mat file using the utf editor which you can dl from this site. Course you could also just dl the texture packs which have all the textures extracted from the mats and in either tga or dds format, again your choice to dl from this site.