The last step is to provide a way for the player to get another chance by clicking the mouse. For this, you need to add some simple code to "mouseDown". When the ball misses the paddle, you set its yspeed to 0 and its y value to something larger than 200. To get it going again, you should set its y coordinate to 0 and its yspeed back to the initial value you chose in "begin". Just add two assignments that make these changes to the body of "mouseDown" and you should be all set.
Test this change and your applet is complete.