Can't place trees with the tree brush - Tried everything

Hi i imported some palm trees from the asset store (https://www.assetstore.unity3d.com/en/#!/content/13540).

I can drag and drop them onto my terrain and everything works fine. But if I use the terrain brush tool no trees appear (only those from the default tree pack)

I heared that you have to put them into an folder called "Ambient-Occlusion". I tried it multiple times but it never worked.

Can someone explain me how to fix this or how and what exactly I have to put in the "Ambient-Occlusion" folder and where this folder has to be?

I got the main Folder "Palm Trees". If i open it i got the folders "Materials", "Meshes", "Prefabs", "Scripts", "Shadows" and "Textures".