Heres a look at the cord going into the XXtra. Its just like it goes into my other two Receivers(all of which use standard Futaba wiring sequence). As you can see the red is going into the signal slot and the black goes to the ground.

All the XXtra manual says is that "if you don't have control of the servos when the DSC cord is connected to the transmitter and the receiver, reverse wiring sequence in the DSC cord input plug plastic and try again." Its sounds crazy, but I guess I'll try it and see how it turns out.