Wireless will be OK if the AirPort Express is located where it can receive a strong signal from the AirPort Extreme to "extend". The reason for this is that the Express can only "extend" what it receives.
I get an error message that says that the selected network cannot be extended.
The usual reasons for this are:
1) The check box to "Allow this network to be extended" in the Wireless settings for the AirPort Extreme is not checked. So the network cannot be extended by any other devices.
Open AirPort Utility - select the AirPort Extreme - Click Manual Setup
Click the Wireless tab below the row of icons
Here is what your settings on the AirPort Extreme should look like:
Wireless Mode = Create a wireless network
Wireless Network Name = Your choice
Enter a check mark next to "Allow this network to be extended"
Radio Mode = Automatic
Radio Channel = Automatic
Wireless Security = WPA2 Personal
Wireless Password = Your password
Confirm Password
2) The Express is located too far from the AirPort Extreme, so it cannot receive a good signal. You need to configure the Express near the AirPort Extreme, then once it is configured and working correctly, move it to a location that is about half the distance from the AirPort Extreme to the area that needs more wireless coverage.
Open AirPort Utility - select the AirPort Express - Click Manual Setup
Click the Wireless tab below the icons
Your settings should look like this:
Click the Wireless tab and adjust settings to look like this:
Wireless Mode = Extend a wireless network
Wireless Network Name = Same name as your AirPort Extreme wireless network
Enter a check mark next to Allow wireless clients
Wireless Security = Same setting as your AirPort Extreme
Wireless Password = Same password as your AirPort Extreme wireless network
Confirm Password
Click Update
As soon as you see the message in AirPort Utility that your settings have been stored successfully, close AirPort Utility. The Express will take another 30 seconds or so to fully power up. Wait for a green light.
If you get a green light, then locate the Express as I instructed....about half the distance from the AirPort Extreme and the area where you need more wireless coverage.