I would have liked to buy some software, but the 'For Purchase' category in the Software Center remains desperately empty.
How do I activate the availability of such apps?
Is it possible that I am unable to connect to Ubuntu One because of my proxy ?
Apparently I am not able to connect to Ubuntu One behind a proxy...
Ubuntu One has problem with proxies since ages, see this bug: https://bugs.launchpad.net/ubuntuone-client/+bug/387308
It seems to also affect "For Purchase" section in the Ubuntu Software Center. I see software listed there when at home (no proxy) but not at work (with proxy).
As far as I know, Fluendo is the only commercial application available as of yet. Keep an eye on it, as more software should be coming shortly!
Currently there are not that many applications for sale in the software centre. The infrastructure is there though and definitely more options should be available in 11.04, and possibly in earlier versions as traction builds. For now, you can enjoy the free software available in the official channels and through ppas.
The "For Purchase" section is filling up, albeit very slowly. Apparently a puzzle game is now available (I can't find it in there, but that's probably because I'm using 64-bit Ubuntu).
As for the steps, simply find an app for purchase, then click "Buy". A window should pop up, asking you to create an account or log in.