You need to activate the engine mouse pointer, add the following
lines in function main after level_load:

Code:

level_load(RTSPlayThing1_wmb);
wait(3);

mouse_mode = 1;
mouse_range = 20000   // play with 20000

while(1)
{
   mouse_pos.x = mouse_cursor.x;
   mouse_pos.y = mouse_cursor.y;
   wait(1);
}