Rotate ball in camera's direction....

I am very new to unity .... I only know that how to rotate a ball using this tutorial .... https://unity3d.com/learn/tutorials/projects/roll-ball-tutorial

But only problem I have is I can go forward,backward, left and right but if i want to change direction....means I want to change direction of rolling the ball ....so how can i do that???

Can you guys help me to find me script....