To connect your Mill WiFi device to Apple Airport Express you have to do an adjustment on your Apple device.
1) Use the "Airport Utility" tool to find your WiFi router
2) Choose your Apple Airport Express device
3) Click on "Edit" in the pop up box that appears
4) Click on "Wireless" in the menu and then click "Wireless Options"
5) Edit "Radio Mode" so that you choose "802.11a/n - 802.11b/g"
Note! Do not change back to "802.11a/n - 802.11b/g/n (automatic)" after connecting your Mill device