Installing the extension is not the last step. Until you allow it, your browser will not let Navis replace a blocked page with its own, so a blocked site shows a plain error instead of the time remaining and a way back to your work.
It is one click, and it does not let Navis read pages, history, or anything you type. Blocking is all it does, and only while a session is running.
Sessions and your site list are managed at joinnavis.com. There is nothing to configure here.