How works Material Painting in Vue 8.4 updated?

I have installed Vue 8.0 xStream + SP1 and Update 8.4 Build 51713.
I followed entire tuorials from AsileFX.
In particular I followed "Vue8 - Terrain Workshop" DVD.
In those lessons the author use the option: "Material painting" from TerrainEditor.
He choose the Mixed material and click StandardTerrain > EditObject > MaterialPainting > Material2 : and check "Apply 2d brushing".

The problem:

• In the Vue 8.4 Update the Material2 option is not available and it is replaced by an "Add" button.
• Why?
• I am little confused about this.

I tried to click "Cancel Last Update" to view the behavior of Material2 Button in Vue 8.0 +SP1 without 8.4 Update;
I have understood that Material2 Button works if it is loaded a Mixed Material in the MaterialEditor for the StandardTerrain.
With "Apply 2D brushing" option I can paint (in example) the Material2 with the EffectTab > "Dissolve" to created the little cracks over the terrain with the second material.
OK!
If I use a SimpleMaterial in the MaterialEditor, by clicking Material2 in TerrainEditor I can choose from the menu window that appears, another material to add it as Material2 in the Material Editor.
OK!

But how this works in the 8.4 Update (instead of Vue 8)?

• How can I do to choose the second material of a MixedMaterial (in example "GrassAndRocks" MixedMaterial) to paint the material cracks with the second material in TerrainEditor?
• If I load a MixedMaterial in MaterialEditor, and then in TerrainEditor I select the "Add" button > "Grass And Rock" (the same as I choosed in MaterialEditor) it appears the following message:

"Volumetric, Cloud and Mixed Materials cannot be loaded as a layer. Please choose another material"

• Why?

The questions are:

1. How can I do to paint the grass as first material, and the rock as second material, from "Grass And Rock" MixedMaterial in Vue 8.4 Update?
2. What is the behavior of "Add" Button in 8.4 Update?
3. With "Add" button can I just load a new material layer, instead of MixedMaterial?
4. How can I do, if I want to switch only from Material1 to Material2, when painting a material, instead of loading another new material?

Please response me!
Many thanks!