By default duplicated objects will be placed on the x-y plane. The center point of all objects will be on zero height. This is not what you need for placing objects on top of a terrain surface. To put objects to the surface of a generated mesh use control input 'of'. Connect this control input with the data channel you are already using for the mesh. This will lift the the duplicated objects on top of your mesh. In addition you can use the offset slider to move the objects in z-direction. Use this if your objects looks like sunken in the mesh.

It is useful to link height scaling of the mesh with the z-location for object placement. The simplest way to do this is by introducing a scaling node. Then scaling can be adjusted by only one parameter in the scaling node. The height of the mesh and the z-location follows this parameter.