OK... here we go:
At the default editor, click up top on the TERRAIN > TEXTURE area... This will open up the terrain texture box.
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Then click on the default (not pictured), and create a new layer. After this, click OUTSIDE of the terrain box (leaving it open) and the hit "M" to bring up the Material Editor. Click on the drop down of Terrain and you will see the following:
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Then click on the material you want to use, and then you WILL be able to use the ASSIGN MATERIAL
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As for it taking over the entire terrain, they have a paint tool for this... The material does not automatically apply itself to your area without you first painting it via LAYER PAINT (if i remember correctly)