These are the steps:
You are doing well assigning the diffuse to the material. This is how your folder should look:
-model.dae
-model.ent (if entity is already made)
-model.dds (diffuse)
-model_nrm.dds (normal map)
-model_spec.dds (specular mao)
-model.mat
If you open it in model viewer, it'll create a material automatically, with the textures you have there (it detects the sufix "_nrm" and it adds it to the material).
However, if you open it in model editor, it will be invisible. That means you have to make your own material:
-Go to the material editor. Then, select the soliddiffuse option (this is for models usually, it might be different in other cases)
-Tick the diffuse, normal and specular boxes
-Assign the textures
-Save the material with the name of your diffuse
It should work now