well i followed that tutorial also. the "is trigger" button means the if it doesn't it something it's supposed to it goes through it (that's why it hits the turret) it is used so the fireballs don't pile up in your way in your game. so nothing is wrong. P.S. here's the turret collision script if you needed it:

var explosion : Transform;

function OnTriggerEnter( hit : Collider ) {

if(hit.gameObject.tag == "name of your fireball's tag")

{

Destroy(hit.gameObject);

var exp = Instantiate(explosion, gameObject.transform.position, Quaternion.identity);

Destroy(gameObject);

}

}