clickable object

Try using this in an update function. Just change the tag to whatever the tag of the object you want to click will be.

 if (Input.GetMouseButtonDown(0))
             {
                var hit : RaycastHit;
                var ray : Ray = Camera.main.ScreenPointToRay (Input.mousePosition);
                if (Physics.Raycast (ray, hit, 100.0))
                {
                      myTarget = hit.collider.gameObject;
                      if(myTarget.tag=="tagOfTheObject")
                   { 
                      //DO STUFF
                   }
                }
            }