In the bottom left window of JCS, click on the little empty tile. Then hold down CTRL and press on !, then an empty block and then ! again.

Buttstomp scenery animations consist of the following:
The first ! block is the one showing up in the level before it's stomped. The second tile (in this case the empty one) is the tile which will be there after the block is stomped. The last tile (!) is the one of which pieces will fall down after the block is destroyed. You can of course insert other tiles, to achieve different effects. Understood? If not I can send you an example level.