Power up the AirPort Express for a few minutes
Hold in the reset button on the Express firmly for 10 seconds and then release
Allow a full minute for the Express to restart to a slow blinking amber light
Connect the Ethernet cable from the switch to the AirPort Express. Here, I assume that you have tested the Ethernet cable connection to verify that it is working correctly.
On the iMac.....open Macintosh HD > Applications > Utilities > AirPort Utility
Click on Other WiFi Devices
Click on AirPort Express xxxxxx
AirPort Utility will take a minute, then suggest that the Express be configured to extend the AirPort Extreme network
Enter a name for the AirPort Express and click Next
Now watch carefully to confirm that the message will change to indicate that the Express is being configured to extend the AirPort Extreme network using Ethernet. (and not using a wireless connection)
When you see the Setup Complete message, click Done. That's it.
Close AirPort Utility and you are all set.