ZModeler Material Help Needed

I could really use some help learning how to make proper GTA IV materials for vehicles in ZModeler. Currently I'm getting the error

Using texture "blahblahblah" which doesn't suit settings for shader "gta_vehicle_mesh".

From what I've read, I believe it means I have invalid materials. But my inexperience with Zmodeler leaves me confused as how I can fix them. Now the following quote is from KevinDV from another topic,

Next you need to consider which shader to use, in this case I will use "vehglass", so then you make a space after your material name as we already have followed by a space and the shader name:

"Windows vehglass"

Now your material name is valid.

Next you need to setup your textures, the basic layout is as follows:

1. yourtexturename - UV1 Modulate

2. dirtmap - UV2 Multiply Add

3. specularmap - UV1 Specular & Multiply Add

Now you're done, if you had a metal material then the layout would differ, a good example would be:

"vehiclemetal mesh" (mesh is used for metal shading and in most cases requires a bump map (The purple 3d textures).)

Your layout:

1. yourtexturename - UV1 Modulate

2. dirtmap - UV2 Multiply Add

3. bumpmap texture - UV1 bump map

4. specularmap - UV1 Specular & Multiply Add

Now this is where I'm just completely lost. I do not understand that UV1 Modulate, UV1 Specular & Multiply Add, and all that stuff. Is it possible for someone to post a screenshot of where those settings are and explain them to me a bit more? I am so lost at the moment. And what is materials like interior2, mesh, tires all about?

And lastly, what is the very minimum amount of parts to make a vehicle? Right now I'm just doing testing with a stockcar model I have and I'm wondering if the lack of doors is going to be an issue and if there's a way around it.