If you add the white glue that ajlafleche mentioned, you should have no problem with the celuclay adhereing. You can improve this by making some holes in the styrofam and pushing the celuclay into the holes that will act as anchors. Some toothpicks stuck into the foam and cut flush with the top of the layer of celuclay will also act as an anchor.
Form most of your terrain feature with the foam. Use a shallow layer of celuclay and you'll get a faster drying time, less chance of the celuclay pulling up or warping and use less material in the coverage. Celuclay warps because one side dries faster than the other. The result is that the topside, drying faster than the underside, contracts and pulls the underside up. The thinner layer of celuclay allows a more even drying.
Celuclay can hold its shape and can be manipulated into some very interesting and complex forms. But, as any soft sculpting media, the longer or taller the piece, the more subject to gravity it becomes and may have to be stabilized with an armature.
I mix my celuclay with white glue and a portion of paster of paris. Throw in any ground detrius like sand, small pebbles or kitty litter, add my water and any pigment and mix it up to a heavy oatmeal consistency. I smooth out any thing that needs smoothing with a spoon or finger dipped in water. You can use this to make mud on wheels or tracks, water, terrain, stucco etc.
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"
