To create a note you first need to select a datasource to view on the Data Viewer panel. Once a datasource has loaded you will see a small blue box called "Notes" with a little + located beside it as seen in the screenshot below:
Clicking on this + will open a new dialog box for you to type your note into. Here you need enter the time you want the note to appear and the information you want on the note.
Once your note is accepted you can view this either on the graph by clicking on the word "Note" or by going into the Events page in the Data tab. You can see in the below snip that our note was successfully created at 10 am and is now visible on the graph for all users to see.