Still got a working Apple AirPort Express and want to configure it using a newer macOS such as 10.12 aka Sierra? Follow these helpful instructions.
The biggest gotcha is joining the correct wifi network name. For me, my reset AirPort Express was advertising a network of “Apple Network”. But if you follow steps 6 and 7, you have to change the name to “Apple Network” and append the last 6 characters of the AirPortID to “Apple Network” so it looks something like “Apple Network 1a2b3c”. Once that’s done, your Mac should be talking with the AirPort Express.
Now fire up the AirPort Utility 5.6.1 Launcher (skip the update notification) and you should see your AirPort Express available for configuration. Made a mistake? Reset the AirPort Express and start over.
On a related note, the original 802.11g version of the AirPort Express does not support bridge (aka client) mode according to Apple.