You must assign the surface to the Surfaces rollout. This is necessary in order to place the items along the terrain, regardless you use an Spline area or not. Please, try this:
1) Assign the surface object in the Surfaces rollout
2) Create the Spline area in the Areas rollout.
3) In the Areas rollout, select the "Surfaces area", and turn it off.
Depending if you have created the Forest object picking a spline or the surface, the "Surfaces area" item is on or off, but you can change it at any time. Usually it must be off when using splines, if not Forest creates items on both areas (stacking them), so the effect of the spline is not visible.
I suggest you complete the Tree-Lined canal tutorial.
It describes the basic concepts of areas and surfaces.