Do not attempt to setup by wireless.. do everything by ethernet.
You might well need to reset to factory and start over. If you are using older version TC or AE.. simply load the old utility.. it is much easier to work with and contains many more options.
If you have the latest AC version airport then you are stuck to using 6.3 which is a dog. No doubt about it.
There are a number of posts in Airport section about setting up roaming on the newer utility.. look for articles by Tesserax who is very good at this stuff, or Bob Timmons.
I will try and locate one for you. Search here is a dog though.. !!
How to load 5.6 into ML.
1. Download 5.6 for Lion.
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
Leave the package open on the desktop so you can see the file. AirportUtility56.pkg
2. Download and install unpkg.
Run unpkg on the desktop.. If your Mac refuses to run the software, because it wasn’t downloaded from the Apple store, go to security in preferences and allow other software to work.. this is limitation of trade methinks. You can set back later if you like.
Now drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. in finder, open the new directory, drill down.. applications, utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
You cannot uninstall version 6 (now 6.3 if you updated) so don't try.. and you cannot or should not run them both at the same time.. although I have had no problems when doing so.
Thanks for your attention. I earlier tried setting up the Express connected to the extreme by Ethernet and was able to get past the Unexpected Error stage. At that time I was trying to set up a wireless extension, but I have given up on that and am now trying to get a Roaming network going. Do I need to do a hard reset on the AirPort Extreme? Could there be a problem with the encryption on the base station making it I reachable by the Express?
Jeez I hate network problems.
BTW: brand new Extreme and Express, mountain lion.
When you run into issues.. factory reset.. go the whole hog.. get rid of everything and start over.
Use names that are fully compatible with everything out there.. ie NOT APPLE NAMES.. so all names are short, have no spaces and are pure alphanumeric.
That simple rule helps no end.
If the Extreme is the new AC version then you must use the 6.3 utility.. make sure you upgrade to the latest.. 6.3.1 and make sure you have ipv6 turned on as Apple belatedly discovered they had made it a requirement.
Be orderly and careful.. write down everything as you go along. Actually roaming network is the easiest to setup.. as there is literally no connection between the wireless in each unit..
So the rules.. apart from the above about naming.
1. Every unit has a different name. TC1, AE2, Ex3, whatever.. short as you can do it with enough to help you know immediately which is which.
2. All of the units except the router are in bridge mode.. start with bridge otherwise you have mulitiple units on the network at the same IP address.. and that is a no no.
3. Do each unit in complete isolation.. plug it into a computer.. name it.. bridge it.. set wireless.. all by ethernet all without any other connection. Update at the end and put it back into the network.
4. Wireless channels set to auto.. is the main point I would part ways with the instructions.. guys here generally say use auto for wireless.. IMHO.. auto is asking for trouble. Wireless is already 80% voodoo.. the less auto the less voodoo gets in your way. You decide wireless channels .. remember each unit has exactly the same wireless name.. exactly the same wireless security settings.. same password.. there is only one thing different.. wireless channel.
There are 3 wireless channels at 2.4ghz that do not overlap.. 1, 6, 11 (ETSI can use 13).
Set each unit to one of those channels.. if you need more than 3 channels well you have too much wireless.. and will have to use mesh system.
5ghz does not have the same issue.. there are more channels and less issues due to its poor range.. just set whichever channels work for you. Remember with AC the 5ghz is important.. that is your fast connection.
I serendipitously found my airport express showing as a "network" on my iphone and was able to configure it wirelessly using the iphone airport utility after getting the usual error message from the desktop utility. I then plugged in and reset my older AE unit and was able to wirelessly configure it to extend my network using the mobile app but not the desktop one. It looks to me like the desktop app has a bug not present in the mobile app which prevents proper function. Using the mobile app everything worked like I was told it would: plug in the AE and it configured itself to extend the network using the mobile version of airport utility. Maybe using the mobile app is the way to get things to work the way Apple said they should.