Simply use the "Import Terrain Heightmap" option in the WorldEditor:
You should see the following menu:
Don't worry about the "Texture Map" fields, it's out of scope of this tutorial. The name should simply be the name of the terrain object, HeightMapImage should simply point to your terrain height map (use the "browse" button).
The tricky part is the "meters per pixel" and "Height Scale", however we could be smart and take care of this when we exported the terrain such that e.g. width would be equal to 2048, then we can simply read the values from the heightmap generator (for example like this in WorldCreator) as the "height" of the terrain, and in this tutorial the relation between the terrain resolution and the terrain width is 1:1 so meters per pixel are simply 1.
Applying the diffuse texture to the terrain
If you want to apply the diffuse texture as one big texture covering the whole terrain (which I personally prefer) then you can do the following:
Add a new terrain layer and set the diffuse texture to be the diffuse texture you exported from your terrain generator and the size to be the resolution of the diffuse texture, this makes it cover the whole terrain:
Now just make sure that every texture layer uses the same diffuse texture setup.