In your landscape material you have to add your textures as “texture sample” nodes -> just drag and drop them into the material editor.
Afterwards use a landscape layer weight node to connect them.
Save the material - assign it to the landscape - go to the paint tab in the landscape editor - klick on the “+” beside the layers - paint.
Otherwise take a look at this tutorial:
https://docs.unrealengine/latest/INT/Engine/Landscape/QuickStart/4/
or take a look at a video tutorial on youtube 