My ball goes the wrong way?

I am working on a game where a ball goes over beams to reach the finish.
It has to be that when I click on “D” the ball goes to the right and when I click on “A”
the ball goes left but the ball goes forward and rearwards.
I’m using Unity 5.
This is the code Where I think there is a mistake:

pragma strict

var rotationspeed = 100;

function Update ()
{
var rotation : float = Input.GetAxis (“Horizontal”) * rotationspeed;
rotation *= Time.deltaTime;
GetComponent.().AddRelativeForce (Vector3.back * rotation);
}

When you can help me pls answere me.
thank you in advance!

AddRelativeForce (Vector3.forward * rotation);

Should do it.