I know this is an old thread but I was able to get the old airport express working with these settings in the image attached - after much trial and error.
1. Hard Reset your airport express
2. Connect your airport express via ethernet cable to your wireless router
3. Use airport utility 5.6.1 to view your airport express
4. click Manual Setup
5. On the main summary screen look @ the channel for the airport express
6. go into your router settings and use the same channel
7. put your wireless router 2.4ghz to WPA2-PSK with AES encryption
8. Save settings & reboot your router
9. in the airport express wireless screen type your network name and password and select WPA2 Personal
10. Update settings on your airport express - as soon as it tells you the airport express is restarting - unplug ethernet cable and give it about 2 minutes to start up and you should see a green light.
I have attached my screens for reference on what I had to do to get it to work.
Thanks to Croey J Mahler for the 5.6.1 launcher - ( I was using EL CAPITAN OS)
http://coreyjmahler.com/note/airport-utility-5-6-1-launcher/
Thanks
Josh