Add .ToString to Score so that Score is converted into a string, allowing it to be written to the label.

 static var Score = 0;

    function OnGUI()
     {
     GUI.Label (Rect (10, 10, 100, 20), Score.ToString());
     }

    function OnTriggerEnter (other : Collider) 
    {
        if (other.gameObject.CompareTag ("Wall")) 
    {
            Destroy (gameObject);
    }
        if (other.gameObject.CompareTag ("Enemy")) 
    {
            Destroy (gameObject);
            Destroy (other.gameObject);
            Score += 100;
    }
    }