I want to make sure that I understand what you are asking.
You have a Time Capsule that is now producing a wireless network signal, correct?
And, you want the AirPort Express to extend the signal from the Time Capsule, correct?
Since there are two ways to extend a network......will the Express connect wirelessly to the Time Capsule......or.....will the Express connect to the Time Capsule using a permanent, wired Ethernet cable connection?
I assume that you have the current version of the AirPort Express.....with 2 Ethernet ports on the device, correct?
What device will you use to set up the AirPort Express.......a Mac, an iPhone / iPad, or a PC? If you have a Mac, I assume that it is running a current or recent operating system, correct?