Didn't we cover how the battery pack would connect to the motor driver already? It will connect to VMOT, and will supply all power for the motors. The 5V VCC connection on the motor driver is just for logic level stuff.
I don't have the Ping, I have the Chinese HC-SR04, but I googled it for you:
First link shows you how to use the Ping sensor. Pretty in-depth.http://tronixstuff.wordpress.com/2011/11/28/tutorial-parallax-ping-ultrasonic-sensor/
2nd link is to a library so you can just do a #include on your programs and issue a simple command.https://github.com/kristopher/Ping
I haven't tried either one, since I don't have a Ping. You may need to find a newer Ping library if that one doesn't work with Arduino 1.0. If you have more questions, I think I'd start another thread in the Sensors forum.