The Ping))) Sensor plugs into Servo Header 10 on the BOE Shield.
Mounting bracket standard servo plugs into Servo Header 11 on the BOE Shield.
Don't forget to center the mounting bracket's standard servo; see the Parallax Standard Servo KickStart[3] download for example Arduino servo centering code.
If you want to, thread a 3/8 inch screw through a hole in the long arm of each L bracket, then through a hole in the top left side of the robot chassis, and secure it with the locknut. Swivel the L brackets so they are 180 degrees apart.
Now you can thread a bamboo skewer traveling staff through the holes in the brackets' short arms.