My set up is this:

My iMac 27" is running OS 10.6.8. An Airport Extreme Base Station is connected to my Internet Modem. The iMac connects wirelessly to the Airport Extreme via its internal Airport Card for general (open) WiFi .

I have also set up the iMac for VPN tunneling. Internet Sharing is set up in Preferences as follows: "Share your connection from - VPN" "To computers using - Ethernet". An Apple TV (original) is hardwired to an Ethernet Switch which is then connected to Ethernet port of iMac. (The switch is for adding additional devices for VPN connection.) Everything works fine thus far.

PROBLEM: I would like to connect wirelessly other devices to VPN Internet Sharing. I am attempting to use an Airport Express (older model A1084) connected to the Ethernet Switch (which is connected to the Ethernet port of the iMac). How do I set up the Airport Express so it is recognized by the VPN Internet Sharing? My Apple TV had no problem since it is hardwired, but I cannot determine how the Airport Express must be configured. I have searched and searched the internet for answers on how to set up the Airport Express but with no success. Thanks for any guidance.