Open blender, delete cube. Don't change the view or 3d point.
Add a plane, switch to edit mode, subdivide the plane, make a uv layout before you change anything of the plane.
Switch to sculpt mode, lock axis x, lock axis y. This way you restrict the movement of the vertices to the z axis.
Now sculpt it.
Export it with ventilator's tool, maybe, fbx works as well, didn't test it.
When moving a vertex or several vertices, restrict the movement to the z-axis, always. That should keep you within the requirements of MED for terrains.
When looking from above in orthographic view the grid of the mesh has to be as regular as a chess board. The height of the vertices doesn't matter.
It will be exported as mdl, now you can change mdl to hmp in MED and save it.
