Ok I understand how to create my own material and add the textures to it. but how do I paint with preexisting materials? Like if I buy a landscape material pack in the marketplace, How do I add them to the paint section so I can use them to paint the landscape since there already made materials.
You will have to add the different parts into your own landscape material + connect them with the landscaper layer weights/blends -> https://docs.unrealengine/latest/INT/Engine/Landscape/Materials/index.html 