Congrats! It seems that you have the first step - the installation behind you! Now to allow your users to add notes you should go to Extensions -> Module Manager and here you should publish the CNotes module. The module should not be published to the general public, but to your registered users!
Once logged into your Joomla backend you can access the component by clicking on Components -> CNotes . You'll be presented with a list of all notes entered by your users. Using the edit & delete buttons at the top you can edit & delete any note you want.
Per default registered users won't be able to create, edit or delete their notes. You need to navigate to the backend of the component and click the Options button on the notes view.
Once you do this you will be able to configure the permissions for each and every group that should have access to the component.
Access Administration Interface - the user that have this permission will be able to access the administrator interface of the component (provided that they can log into your Joomla backend)
Configure - the users that have this permission will be able to access the component settings/options
Create - users that have this permission will be able to create notes in the frontend
Edit.own - users that have this permission will be able to edit notes in the frontend
Delete.own - users that have this permission will be able to delete notes in the frontend
The user who is accessing the administrator interface needs to have the edit.own & delete.own permissions in order to edit & delete any notes in the backend.
Super users can always edit & delete notes.