The Y input to the Cartesian variety of the Mecanum Drive vi follows the standard HID joystick convention where negative means forward. If your joystick value is negative but your robot is going backwards, you probably need to change the MotorInverted inputs on your Open Drive vi in Begin.
If your joystick value goes positive when you press the joystick forward, the appropriate fix is to insert a Negate function between the joystick axis output and the mecanum drive Y input. It looks like (-x) in a right-pointing pale yellow triangle, and it can be found in the Numeric function palette.