How do I teach my dog how to target the ball when using a ball on a string?

Do you have any training or suggestions to train my dog how to go for the ball and not the rope when using the ball on a rope toy? I have a Rottweiler with a huge toy drive. I would like to use the ball on the rope as a training aid for reward when training him. Right now, he keeps going after the rope and not the ball. I purchased the ball on a rope with the handle at the end from your site. I was hope you can point me in the right direction. Thanks.