Joints are intended to create some specific physics behaviours - they don't modify the object mesh. You can achieve the effect you want with a simple script (attached to the cube):

var ballA: Transform; // drag sphereA here
var ballB: Transform; // drag sphereB here
var scale0: Vector3; // initial localScale

function Start(){
  scale0 = transform.localScale;
}

function Update(){
  var pA = ballA.position;
  var pB = ballB.position;
  transform.position = (pA+pB)/2; // place the cube in the middle of A-B
  transform.LookAt(pB); // make it look to ballB position
  // adjust cube length so it will have its ends at the sphere centers
  var scale = scale0;
  scale.z = scale0.z * Vector3.Distance(pA, pB); // stretch it in the direction it's looking
  transform.localScale = scale;
}

This works fine for a simple Unity cube, because its localScale is numerically equal to its dimensions, but may produce bizarre results for imported objects. A more general, sophisticated and complicated solution would be to use bones (take a look at Mesh) in the docs)

You have to make a physics material and try a friction around 1.

Got it from here:

http://answers.unity3d.com/questions/58794/getting-realistic-bounces-using-physics-materials.html