Are you using 6.1.13.1?To find the version, hold down the Control key and select About Bryce 6.1 from the Help menu.
Also what version of Poser?
And as Bobby suggests, open the .MTL in a text editor and make sure that the path to the textures is pointing to the right place.
For example, the .MTL statement:
"map_Kd MyHardDrive/Users/danielwhiteside/Documents/bulidings/slat_facade_2.psd"
is the path to the diffuse texture map.