Right click to the Outline section and click to the Add terrain button.

The terrain created with Terrain name.

If you select it with left click then you can see the terrain in the on the editor.

You can generate heightmap after selected the terrain. You see the settings on the Height map tab on Gen tab. Here you can select the height map image.

If you select a heightmap and click to the Load heightmap button than it will modify the selected terrain.

You can generate Perlin noise after selected the terrain. You see the settings on the Perlin noise tab on Gen tab. Here you can set the Seed, Min height and Max height.

If you set these values and click to the Generate Perlin noise button then it will modify the selected terrain.

You can change the texture of terrain on the Paint tab. Right click to the default texture and click to the Change texture button.

Select an already imported texture.

After selected the texture the texture of terrain changed.