There is a link on that page to a Word document that describes how to connect the device to the motors, the batteries, and the micro-controller (the Arduino). There are instructions regarding how to select which of 4 control modes to use.
To start with, select analog input mode, and select two PWM pins to connect to.
Programming the Arduino to output the proper PWM value is so simple, I'm going to leave that to you to figure out.
5- Design a data command structure to send to the Adriuno from the PC the control the Bot.
What data do you need to send? There are only two inputs needed by that shield - a byte to control the speed of each motor. The "command structure" simply defines whether the first value is for motor 1 or motor 2. The other value must, then, be for the other motor.