Create yourself a cube that is not rendered and flagged as a trigger only. Place this cube where you want the ball to be "affected" by the air jet. In your ball gameObject, attach a script which has code similar to the following:

function OnTriggerEnter(other:Collider)
{
   rigidbody.AddForce(Vector3.up * 20, ForceMode.Impulse);
}

What this code says is that when the ball enters a trigger area, an instantaneous impulse force will be applied in the "up" (Y Axis) direction. I tried this out and it worked like a champ.