OK, I assume that the Time Capsule is producing a wireless network signal. That is critical.
Please follow these instructions very carefully.
Temporarily, move the AirPort Extreme to the same room or close proximity to the Time Capsule.
Make sure that the AirPort Extreme has been powered on for a few minutes
Hold in the reset button on the back of the Extreme for 5 to 6 seconds, then release
Allow a full minute for the Extreme to restart to a slow, blinking amber light status
Click the WiFi icon at the top of the Mac's screen and look for a listing of New AirPort Base Station
Click directly on AirPort Extreme under that heading

The setup "wizard" will open up and take a few seconds to analyze the network, then display a screen that looks like this....except that you will see your Time Capsule and AirPort Extreme pictured.

Type in a short simple device name for the AirPort Extreme. Mine is AEBS, for example
Click Next, and the setup wizard will configure everything for you automatically.

Allow a minute or two and the next screen should say Setup Complete.
Click Done
Move the AirPort Extreme back to a location where it can pick up a strong signal from the Time Capsule and you should be all set.