Hi nextpuma,
What Juan Ramón wanted to say is that it's not yet supported on OSClass and you should add your custom rules on a .htaccess files or better, into generate_rules.php (located at main folder, and later run it).
Will add support to it soon, but since there're many possibilities, it's complicated to fit all and make everyone happy.
For example, your could think that osclass.org/city is ok, but other could wanted to be osclass.org/category (as it's at the moment) osclass.org/country or region, or any other field are options that maybe someone would choose. Also, we need to think how to handle multiple params, as city+category for example.
We're working on that right now.
Thanks