To load a plugin you must add the loadplugin command in the configuration file. For example:

loadplugin pad.plugin.pyzor.PyzorPlugin

If the plugin is not located in the python path then you can also specify the full path to the file:

loadplugin MyCustomPlugin /home/pad/my_plugins/

Some plugins are reimplementing existing ones from SA. The full list can be seen in pad.plugins.__init__:

loadplugin Mail::SpamAssassin::Plugin::Pyzor