Ok, let me with my bad english give a try about the texture.
First I suppose you have made a terrain in your map, haven't you?
The texture you have choosen for your terrain is saved as Alphamap in your MyLevel-package.
Do you know Mylevel-package and can you find it?
If you can find it you will see one texture named Alphamap and one named Heightmap, but they are both just grey, ok?
The grey texture named Alphamap is the one you have to use in the decolayer.
No, wait a minute...I'm wrong now.
I made my terrains from an old tutorial and it was tolding that you have to save the terrain-texture as Alphamap in Mylevel-package...but I can see in Rachel's tutorial that you don't have to.
So you must have saved your terrain-texture in your own name in your own texture-package for your map, haven't you?
Then open your own texture-package and locate the terrain-texture...but that texture should be grey.