At this point I would recommend at least two things: 1) Perform a "factory default" reset on both base station to be sure they are in their "out-of-the-box" configurations, and 2) Temporarily connect the AirPort Express to one of the available LAN ports on the Extreme during the setup phase. Once the setup is successful, you can remove the Express and place it at the desired location.
For reference on how to setup an extended wireless network, please check out this Apple Support article on the topic.
I have a similar prob but more basic! Straight out of the box, I get 6753 error when the airport set up is trying to read the new time capsule. this happens with either my new Mac pro I bought 2 months ago or my girfriends old mac book air.
Stumped! I get to the 2nd step in the support article before I'm out of options.
I'm an ex PC user, so maybe I am still adapting to how "easy" this is meant to be and missing something really basic?
Thnks for any help!
A google search revealed:
The solution to this common problem is: enable IPv6 in your wireless network settings (on your computer with AirPort Utility). You will be able to connect to your device. It is because fresh AirPort Extreme (or Time Capsule) use IPv6, not IPv4.
It took me three hours to figure this out. However it is 100% working solution in my case: no IPv6 — no connection to a clean AirPort Extreme or Time Capsule.