salmon, first you delete the .txt from the name of the file.  .mtl is a material file.  You'll want to put it in your runtime under materials.  So poser - runtime - library - materials.

Then in the material room you can double click on the file to load it.

Now if you want to keep a preexisting texture file, you might want to use it with VSS instead.  Do a search on VSS for discussion on how to use it.

WARK!

Thus Spoketh Winterclaw: a blog about a Winterclaw who speaks from time to time.

 

(using Poser Pro 2014 SR3, on 64 bit Win 7, poser units are inches.)