As far as I know you don't have to import the .mtl file. The .obj should read it by default. It may happen that the .obj file is not linked to the .mtl properly. Try checking the file in Notepad to see if the path is correct.
I've noticed that textures don't hold too well in an .obj export/import. So I usually don't bother. FBX should give better results, but that's not of much use if you're trying to import an .obj, I suppose.