How to do Layered Materials in Unreal engine 4 UE4

#1

So I Just watched the fallowing video on layered materials in unreal 4

//youtu.be/So%20I%20Just%20watched%20the%20fallowing%20video%20on%20layered%20materials%20in%20unreal%204%20%20<iframe%20width

However I just don’t understand one thing about it. I understand how a Mat-layer blend node works, and how a material ID works to give coordinates to different layers. However What I don’t understand is how they dragged materials in to other materials. I mean I tried dragging a material in to another material in the engine and nothing happened. Any ideas guys?