Airport Time Capsule won't join existing network

I've spent the better part of two hours trying to get it to join our network. When I click on "Other Options" and select add to existing network, the next screen tells me it will create a network and asks me to name it, our base station, etc. I've reset it numerous times, tried connecting wired and wireless to fix this and so far nothing.

What am I doing wrong?

Mysteriously, this morning, the created network has internet access. Last night I could get it to create a network but it would not access the internet.
Last night it would not, and still won't, join an existing network.
I guess I should be happy I can at least back up my Laptop.....

For what I believe you are trying to do...

You have another router handling DHCP in the house...

The Internet tab should be set to Connect Using: DHCP
The Network tab should be set to Router Mode: Off (Bridge Mode)

Then give a try selecting the Extend a wireless network option.

In a sleep deprived stupor last night I set up a call with Apple support. They called me this morning and it turns out my router/modem and the Airport TC speak differing 802.11 languages. The Airport TC can't join my network because of this issue. I am able to back up my laptop and access the internet. I was advised to disable the wireless network on my other router/modem as it can cause interference problems in the Airport TC network it created. Right now I am not having any that I notice but we'll see how things go.