Hello,
One thing to keep in mind, the models included tend to be scaled very small. Try scaling them up using the SCALE OBJECT command. The scale factor is in percentages so 200 would be 2 x the size where as 50 would be 1/2 the size. Just keep sizing them up until you see them.
As far as whether they work or not and how the texture relates, the non textured models work but they should contain a definition inside of them that is called Material. This is a color definition and can be applied to the whole mesh or individual faces. If there is a bitmap associated with the Material, it will be defined as a Texture. The Texture will override the color definition contained in the material - (but alpha blending can still be applied).
Here's an example of a Material definition:
Material bone {
0.900000;0.622900;0.227800;1.000000;;
200.000000;
0.461200;0.419300;0.267300;;
0.000000;0.000000;0.000000;;
}
This color is a sort of orangish in this example.
Here's a clip from the DBC manual:
If your model file contains texture information, you need to make sure the texture bitmaps are located somewhere within the same directory as your model file. You should also be aware that the X file format only supports bitmap texture filenames in the 8.3 format. This means your texture filenames should be truncated to its first eight characters and end with a '.bmp' extension.
Enjoy your day.