Once you've installed the plugin you must load it on cssed to make it work. cssed will remember what plugins was opened last time you closed cssed, so once it's loaded you have no need to load it again on each boot.
To load the plugin you must select the "Plugins" menu, then the "View Plugins" sub-menu, and it will be shown a window with all the plugins available to cssed. The python plugin is "pythonscript.so", if it's not shown in this list something failed in your installation, or you did not installed the plugin.
If it's show you must select it and hit the "Load" button. If no errors aroused you're done.
Once you loaded the python Plugins, you'll have available its menu on the main menu bar "Plugins" menu, and also the "Python scripts" entry in the editor drop down menu.