Looking at the picture of the export optopns, above in this thread, try:

*  UN-checking "Full Texture Paths"
* CHECKING "Copy textures to destination folder"

   This will export a copy of the texture files alongside the exported OBJ, and reference them with no path name in the .mlt material file. The textures, .mtl material file, and OBJ must remain in the same folder with this arrangement. After doing this, try opening in the Windows model viewer again.

If this works, then there is some pathing name problem with the exports when exporting "Full Paths" (Like, the programs can't find the textures where there were after export). If this still doesn't work, then observe  *IF*  it really did export texture files along with the OBJ at all... If it did, open the .mtl file in a text editor and just check that the filenames of the textures are, in fact, correct.