Random instantiate script

Your question wasn't quite clear, but I think that you want a ball to move up the screen but start at a random point on an axis?.

If so, then try this script:

var fireRate : float;
var ball prefab : GameObject;
private var lastShot : float = 0;
function Update()
{
  if(Time.time > lastShot) {
    var go : GameObject = Instantiate(ballPrefab, transform.position, transform.rotation);
    lastShot = Time.time + fireRate;
}
}

Then just add a constant force to the ball, and a simple ping pong movement(via animation) to the empty game object the above script is attached to.