A tip when using scatter, if you want the scattered objects to be within a certain area (over a terrain, for instance), start with 4 copies of the object you want scattered. 4 rocks, 4 trees, 4 spheres, whatever. Place one object at each "corner" of the target area. Select all 4 of the objects, then move to the Scatter feature and check "use selected objects." Doing this will set the extreme distances for you so all you need to fill in is how many to scatter within that area. :)