I tried txesmis method by using parts of the code and I think its working: I am getting a green dot where my mouse is pointing, anywhere on my map.

I altered two things in the code:
1)
Quote:
draw_point3d ( vecMouseGround, vector(0,255,0), 100, 30);

because my Gamestudio seems not know COLOR_GREEN (Maybe because I am using 7.5 Extra Edition?)

2) Since I put my terrain in the level using WED,
Quote:
if ( vec_mouseground ( vecMouseGround, terrain.z ) )

is also not working because there is no Entity or something called terrain (I am only speculating this, might be wrong).
So I just changed it to
Quote:
if ( vec_mouseground ( vecMouseGround, 0 ) )

because my WED-Terrain is at the position x=0/y=0/z=0.

Now the only thing I am trying to figure out is, how I can get the coordinates on the green dot into another command.
Like rotating my entity towards them and then moving my entity there. Maybe I am just really silly, but I can't use
Quote:
vecMouseGround
in the right way.
with
Quote:
c_move (my, nullvector, vector(vecMouseGround.x * 0.02 * time_step, vecMouseGround.y * 0.02 * time_step, vecMouseGround.z * 0.02 * time_step) , GLIDE);

my Unit also moves in very weird ways.