Hi! I was having this same problem and I think I might have worked it out. Apparently there is a limitation in scale when using Heightmapper. If your map is of a relatively small area, it will not show the gradient map, just a blank page as you said. However, if you zoom out (using the minus button in the upper left) eventually you should start seeing some texture.
I suggest you try again by zooming out in google maps and inserting the link of this now broader area. It might not give you the detail you are looking for, but it will give you something to work with. You can then crop and adjust the map in Photoshop.
Hope this helps!