For 2004 models here is the .mdl editor called mdlmat, when you get to the page to view mdlmat and you see the yellow dot next to mdlmat click on that for the download. Also needed will be from the home page is the Latest version of the mwgfx DLL package to run the program(s). http://www.mwgfx.co.uk/index.htm
For a FSX version of the .mdl editor Model Converter 1.3 http://fsdeveloper.com/forum/showthread.php?t=14215
The .mdl file I couldn't open with the 2004 editor opened in the FSX Model Converter 1.3 editor then. These programs require some reading on your part to understand how they work. The 2004 I figured out rather easily on my own, pretty simple program. The Model Converter 1.3 is a far more complex program. After time this program once figured out on my end is capable of changing certain items on models created with GMAX and a few other programs.
These helped me alot to paint parts that were not on the .bmp or .dds files anywhere. This was why I used these programs. I am currently working on trying to restore a alpha channel on a aircraft where it was not present to begin with the Model Converter 1.3