I can't shoot up or down. HELP!

i can only shoot straight forward, not up and down! Script;

var projectile : Rigidbody;
var speed = 20;

function Update () {

if(Input.GetButton(“Fire1”)) {

clone = Instantiate(projectile, transform.position, transform.rotation);
clone.velocity = transform.TransformDirection(Vector3(0,0,speed));

Destroy(clone.gameObject,3);

}}

try with.

clone.rigidbody.AddForce(transform.forward*force);