devilkkw's answer shows the right thing, but saying "simple code" and then post a script with 98% unrelated stuff is not really a good answer.

What you actually need, i guess you've found it already, is Input.GetAxis("Mouse ScrollWheel"). It returns the delta movement that happend in the last frame. It can be positive or negative (up/down).

Make sure that the axis is setup in the input manager and named accordingly. The default "Mouse ScrollWheel" is of type "Mouse Movement" and uses the 3rd axis which is normally the scrollwheel.

this is simply code to do:

    var target : Transform;
var distance = 10.0;

var xSpeed = 100.0;
var ySpeed = 100.0;

var yMinLimit = 2;
var yMaxLimit = 80;

var distanceMin = 3;
var distanceMax = 25;

private var x = 0.0;
private var y = 0.0;





function Start () {
    var angles = transform.eulerAngles;
    x = angles.y;
    y = angles.x;


    if (rigidbody)
        rigidbody.freezeRotation = true;
}

function LateUpdate () {
    if (target) {
        x += Input.GetAxis("Mouse X") * xSpeed * distance* 0.02;
        y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02;

       y = ClampAngle(y, yMinLimit, yMaxLimit);

        var rotation = Quaternion.Euler(y, x, 0);

        distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel")*5, distanceMin, distanceMax);




        var position = rotation * Vector3(0.0, 0.0, -distance) + target.position;

        transform.rotation = rotation;
        transform.position = position;


    }

}


static function ClampAngle (angle : float, min : float, max : float) {
    if (angle < -360)
        angle += 360;
    if (angle > 360)
        angle -= 360;
    return Mathf.Clamp (angle, min, max);
}

attach on our camera and select target.