Beginner's Frustrating Experiences with the Engine

You can paint the landscape by Ctrl + clicking.

Here’s the documentation on Paint Mode:
https://docs.unrealengine/latest/INT/Engine/Landscape/Editing/PaintMode/index.html

You’ll also most likely need this one on Landscape Materials:
https://docs.unrealengine/latest/INT/Engine/Landscape/Materials/index.html

I hope that helps.