Login to phpMyAdmin or whatever script you use to view your database. If you do not have access to your database ask your host to provide the exact name of your database. With my host they add dkiefer_ to the beginning of my databases so I have a database named dkiefer_photopost which in config-inc.php looks like:
// PhotoPost's database name
I then prefer to add a prefix to the tables for each of the PhotoPost products I install so for PP Pro I add the prefix pp_ and in the config-inc.php file it looks like:
// this variable after performing a default installation.
$pp_db_prefix = "pp_";
This makes my tables inside of dkiefer_photopost look like pp_settings or pp_photos and so on.
If you look at phpMyAdmin you should be able to see what your exact database name is and the prefix for Classifieds. If you cannot figure it out I would suggest purchasing the install service we provide so one of our installers can help you find the problem.