That
usually means you have the folder structure incorrect. Using Flagg Park as an example, you should have the following structure:
Code:
ballparks
models
Flagg_park
Flagg Park_Textures
diagram.png
Flagg Park.mtl
Flagg Park.obj
Flagg Park.ootp3d
Flagg Park.skp
flagg_park.prk
flagg_park_d.png
flagg_park_n.png
The .obj and .mtl files work hand-in-hand. The .obj file describes the actual 3D structure, and the .mtl file provides information about the textures. The textures have to be in the right location relative to the .mtl file.