How do I add a pulling force?

Assuming that the cubes have dynamic rigidbodies, first find the direction from the cube to the ball like this (assuming that the cube that is to be pulled is stored in a GameObject variable called "cube"):

 if (Input.GetButton("Pull")){
     Vector3 direction = transform.position - cube.transform.position;
     //And then we give the direction a length of 1
     direction = direction.normalized;
 }

Next you would move the cube towards the ball like this (assuming that it's stored in a variable called "cubeRb"):

 cubeRb.AddForce(direction * 20);

The 20 can be changed to whatever number is needed, and this line of code needs to follow after the "direction = direction.normalized;" line. Anyway, I hope that this helps!