Go to your Joomla backend -> components -> CComment -> settings -> new, from the component dropdown field select com_matukio and click next. Save the configuration.
Now go to Extensions -> Plugin Manager -> find the Content - CComment plugin -> edit it and set the "Enable for com_matukio" parameter to yes. Save your changes. Now in the upcoming list view in matukio you should see the "write comment" button and in single view of an event (in the modern template) you should have the comment form.