The option to join is no longer included I think in v6 utility if you are unfortunate enough to run Lion.
Download and install a real tool utility.
Hold down option key when you choose wireless mode and select join.. you can always join TC to the wireless... it will then not extend, turn off ethernet so no bridge.. but it will be part of the network.. it is missing because Apple consider it bad.. as indeed do most people who use it.. slow poor way to do things.. because apple decided to only provide repeater to their own products.
Posting new threads just makes it harder. This might have already been suggested but I refuse to go back and read through now two other threads. You are not helping yourself at all.