How to set the mouse x and/or z position as a variable

I just wanted to ask how to get the x and z position of my mouse cursor. I need for my rts-style game. I have a button and when the player presses it, the game initiates my house prefab. But i find myself unable to create it at my mouse coordinates. I only can create it at some defined points ( 0, 1, 0).

My code looks as follows for the moment: var house : Transform;

 var z = RaycastHit.point.x;
 var x = RaycastHit.point.z;
 
     function OnGUI () {
             if (GUI.Button (Rect (10,10,100,30), "Domek")) {
                  Instantiate (house, Vector3( x,1, z), Quaternion.identity);
             }
     }