Okay, a normal is a direction vector (length 1) which points orthogonally away from a surface. Now a vertex is part of one or more triangles. Since these triangles usually don't point to the same direction the vertex normal is the average of the triangle's surface normals. That way edges look smooth (needed e.g. for spheres). For a box (or in your case the wall) you don't want to smooth edges, you need "hard" edges. That means that a vertex that is part of multiple triangles must be replaced my multiple vertices that are part of the different triangles so they can all have different normals.

I don't know how to do that in MED since I don't own any Gamestudio anymore. But another way to achieve what you want is to set a material with shade mode set to flat. Iirc there's a predefined material.