UE4-The meshes material not showing

Hello,im new in the UE4,and i downloaded a assets pack,to start making my own landscape,i copy pasted the assets files from rar. to my UE4 project folder,and now when i launch Unreal Engine and try to place a tree or something else in the meshes folder,the object don’t have color
As you can see in this picture:

So,can someone help me??I’m new in the UE4,so i want a detailed explanation,how can i add a material to this tree?