Like any enrolment plugin, you have to add it to the course you want to use it in. Please see the documentation on how the course enrolment plugins are used and work.
This plugin is very similar to the Moodle standard Self-enrolment plugin in the way it works, in general. But if you have questions specifically on how this plugin works, you should look on the page for this plugin to see where the maintainer of the plugin answers support questions.