I don't know what kind of an effect your going for. I assume in a pong game when the ball hits the paddel it get reflected off in the opposite direction that in came in, can't tell you how to do that based on your code.

I'd detect the collison by measureing from the center point of the ball and compareing it to the distance from the regtangular area of the paddle. If the difference is less then the radius of the circle you have a collison. The math for that can be found in most gaming books.

Its hard to give you an exact answer based on your code.