We assume that you have a new Airport Express with factory default settings that has not been set up previously.
Locate the AirPort Express about half way between the AirPort Extreme and the Apple TV and power it up for a few minutes. Location and placement is very important. The more that you have line-of-sight between the AirPort Extreme and AirPort Express and Apple TV, the better the network will perform.
From this point, we are simply following the instructions in the Setup Guide that came with your AirPort Express.
On the iMac......click the WiFi icon at the top of the computer screen
Look for a listing of New AirPort Base Station
Just below that, click directly on AirPort Express

When you click on AirPort Express, that will get the setup "wizard" running. It will take a few seconds to analyze the network, then present a window that looks similar to the illustration below, except that you will see your devices and the name of your wireless network.

In the box next to Base Station Name, type in a short simple name that you want to call the AirPort Express
Click Next
The wizard will set up everything for you automatically. When you see the message of Setup Complete, click Done and close the windows.
Pull the power cord from the back of the Apple TV, wait a few seconds, then plug the power cord back in and give the Apple TV a few minutes to power up and restart. You should be all set.