Connecting to an existing wireless network using Time Capsule

I am getting a "status" problem with trying to utilize my time capsule on my existing network that states "extended network". It also states "This Airport Base Station was unable to extend the wireless network. Make sure the wireless network you're trying to extend was setup and is working correctly, that you entered a matching wireless network password into this device, and that this device is in range of the wireless network." The wireless network is and has been setup for the past couple of days through Comcast cable. I have checked and rechecked the password 20+ times and I know for a fact the device is within range, about 10 meters away. I have performed a "hard reset" on my time capsule in hopes that would fix it, I have also powered down the Comcast modem multiple times throughout, but to no avail. All I want is to connect my Time Capsule to the existing network as a secondary WAP to push the network connection further and as well to allow my Mac Mini to back-up with Time Machine. Any help would be greatly appreciated, thank you.