Please bring the AirPort Extreme back to a location near the Time Capsule and look on the bottom of the device to locate the model number. It is usually embedded in the foam base.
The model number starts with an "A" followed by four numbers. Post back with that info.
Next, we don't know......and you do not indicate what device that you are using to try to set things up. If you are using a Mac, click the Apple icon in the upper left corner of the screen and then click About This Mac and post back with the OS X Version that you see there.
Finally, the attic is not likely the place where you want to install the AirPort Extreme. It is likely very hot in that location, and the AirPort will need to pick up a strong signal from the Time Capsule if you want the it to be able to "extend" or "repeat" the signal. The normal location for an extending device is a point that is about half way between the main router....your Time Capsule in this case.....and the general area where you need more wireless signal coverage.
In other words, if you need more signal in an upstairs bedroom, the AirPort Extreme would be located about half way between the Time Capsule and the bedroom.