It is not a function provided in v6 utility.
Why?? Because Apple decided you don't need it. And what Apple decides is for your best interest.. you just say thankyou sir for making all my decisions for me.. it makes life easier when I am ruled .. etc.
It is slow.. and the TC will be out of action as far as backups while it does it. But IMHO there was no need to scrap it.
Apple have a set of instructions on backing up the TC.. with USB plugged into the computer.. this is actually much faster.
http://support.apple.com/kb/HT5096
The best way if you do still want to use archive.. and lots of other things that got eliminated in v6 toyland utility.. is to install v5. This is not that difficult even on Mountain Lion.
How to load 5.6 into ML.
1. Download 5.6 for Lion.
http://support.apple.com/kb/DL1482
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
2. Download and install unpkg.
http://www.timdoug.com/unpkg/
Run unpkg on the desktop.. it is very simple.. drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. 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 6.1 so don't try.. and you cannot or should not run them both at the same time.. so just ignore the toyland version.. the plastic hammer.. and start using 5.6.. a real tool.
For screen shots see this post.
https://discussions.apple.com/thread/4668746?tstart=0
I have both available on the Mountain Lion MBP I have.. I only use the v6.1 to answer support questions though.. as it adds nothing and is a subset of v5 tools.