You've built a totally different robot from the one shown in your link, using a completely
different set of parts, and you wonder why it doesn't work.
And on top of that, you've not even looked at the dozens of descriptions of how to use
the following devices on the main Arduino site and Playground,
Are there any instructions on using Arduino, XBee, joystick, and servos, or Arduino, XBee, joystick, motors, and motor driver?
What's wrong with this picture?