[DavidGitz]

Besides what Alan said, here is how we make sure the wiring and software are correct, assuming you are using 2 Joysticks for control, one for forward/backward and strafe left/right and one for rotation:

1. Put the robot on blocks so the wheels are free to rotate.
2. On the Driver Station, make sure that Joysticks 1&2 are assigned to the correct port in the Setup Tab: We prefer that the strafe Joystick is #1 and on the left, and the rotate joystick is #2 and is on the right. Click any button on the Joystick and that will tell you which one is which, and you can click and drag the joysticks to re-order if you need to.
3. Enable your robot with the Mecanum code running. Slowly move Joystick #1 to a small value forward and hold it there. Check to make sure all the wheels are rotating and rotating in a forward direction. If any wheel is rotating the wrong direction, either change the wires or change the motor inverts in the Begin.vi
4. Go forward and backward full range to verify that everything mechanical is rotating correctly without any binding.
5. Use Joystick #2 and rotate to the left. This should make the left side of the robot's wheels rotate backwards and the right side move forwards. If this is incorrect your speed controller's PWM cables or software is using the wrong motors.
6. From what we've been able to tell, if all the above steps pass the robot should strafe correctly. So put it on the floor and verify that it moves correctly still. If there is a problem with the robot now, it is more likely a mechanical issue, such as binding, wheels are not touching the ground and/or bad weight distribution.