How do I add a new target layer that I've downloaded to my landscape?

I downloaded “Forest landscape material” from the marketplace and I want to use it to paint my landscape. I opened my landscape and in “paint” tab I took a screenshot:

It currently has 4 layers under “target layer”. I wanted to add more, so I tried to move the layer info from the content folder to the layer tab, but it doens’t work. How do I add the layers from the content folder to be able to paint my landscape?