I couldn't find a fritznig model for the Seeed motor driver, so that's implied as atop this one. In this you'll see the Netduino and how it drives the ping. The Netduino does get power from the motor driver, but it doesn't drive the ping sensor at all. Seems like re-routing some additional channels (VIN, maybe?) to the breadboard and back again might make things work better for the Netduino?
I also attached the best photo I could find of the shield for your review. The Netduino runs from 5V/GND/Digital to the Ping. Works just fine so long as the Netduino is plugged into USB.
Thanks for your review and assistance!