How to add colliders to a character controller

I just got unity and iv been trying to make a sphere move around on a platform but i have 1 problem. The sphere just falls through. This is a problem because I dont know if my scrpt is working. I have a sphere collider on my sphere and a box collider on my platform. If anyone has input please help me!

you need to add Rigidbody Component also.

http://unity3d.com/support/documentation/Components/class-Rigidbody.html

I added a rigidbody and it is still falling through the floor, I have a mesh renderer a sphere collider a rigidbody a character controller and a script on the character controller! help please

var speed = 3.0;
var rotateSpeed = 3.0;

function Update ()
{
var controller : CharacterController = GetComponent(CharacterController);

// Rotate around y - axis
transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 0);

// Move forward / backward
var forward = transform.TransformDirection(Vector3.forward);
var curSpeed = speed * Input.GetAxis ("Vertical");
controller.SimpleMove(forward * curSpeed);

}

@script RequireComponent(CharacterController)