An alternate solution might be to render an "altitude map" of your terrain from top view and export the grayscale to an image editor and "paint" in the footsteps. I used to use this method quite a bit in Bryce and I've discovered that it can work very well in Carrara also. It can also be cool if you do a topview render of your terrains' textures so that you can "bake" them in (you might want to disable shadows for this) and apply the bitmap to your your greyscale generated terrain. This bitmap can also be detailed in the image editor to match the altitude map; it's a very good way to add roads and other details to terrains.