In "WPI_RobotDriveMecanumDrive_Cartesian.vi" (accessed by double clicking on the "Drive -- Holonomic Drive" block in the Teleop vi) over on the far left toward the bottom you see "Rotation", "X", "Y" inputs. By rewiring which goes to which input, you can control which joystick motion/wheel controls what behavior.
Please see this image, this is what works for us:
http://pho.to/8fJab
We use the z-axis input for side-to-side motion. On our old joystick that's a little wheel just below the handle.
Neil
P.S. Also, don't forget to swap the wires for motors 1 and 3.