We checked with Apple, and the explanation is that the older model AirPort Express, Model A108x, supports only 802.11 b/g. One must have a newer model, A1264, which supports 802.11n, to configure it with AU 6. This limitation also extends to the older AirPort Extreme without the “n” capability.
Users of OS X Lion can use the older AirPort Utility, version 5.6, to configure “g” Airport devices. Snow Leopard users can use AirPort Utility 5.5.3.