Are you loading to the correct Materials section?  I have mine this way:  Poser 5/Runtime/libraries/materials/file name/item.  The item consists of two files a .mz5 file and a 91X91 pixel .png. 

All the Material room files that I have (except stock) are created by me WITHIN the materials room.  For example, I have a stock V3 (blend texture very blend).  Then I node up and get the image texture, then doodle a bit with the other channels.  Then I add the mixture to the materials room library via the controls on the right side.  That creates the .mz5 file along with that .png postage stamp. 

What the materials section  does (IMHO) is package your material room channels, nodes, etc, so that you don't have to re create the mess every time you create your figure, gem stone, archway, building etc.  Jan