All the tutes and help stuff (the most comprehensive and useful help file in the universe...No Joke!!) state that I don't have to actively import a .mtl file with an imported .obj file.
I use Blender for my 3-d object modeling. I am having trouble getting Moho to recognize that the extra .mtl file exists, even when it's in the same folder with the .obj file!
Basically, Moho only recognizes the diffuse color component of a material in a MTL file, or mapped images. Anything else, particularly including any procedural materials, is gonna be ignored.
If you have problems with the diffuse color component that 7feet mentioned you can open the mtl file with notepad and just make sure the paths are right.