How are you connected to the TC??
I think it is always better to do the setup via ethernet. That provides a fixed link.. plug it into your imac.. doesn't matter if the setup is in isolation to the rest of the network.. turn off wireless in the imac.. plug in the TC.. start the airport utility.. if it still cannot find the TC.. press reset .. 10sec around about and start over.
If you still have no TC.. take it back for checking.
I forgot a couple of things..
What OS are you actually running on the computer? What version utility?
For v6.3 utility to work it needs ipv6 turned on.. is it?
There is no doubt about it.. v6 was a backwards move.. and 6.3 is worse than anything that came before.
People are having much better success with airport utility on the iOS.. !! Even if it doesn't have the same level of info it works much better.