Clicking the Notes icon in the Session Info section opens the Notes popup, is this option has been enabled by an Administrator as described in Note Taking.
To add a note, type the note in the field and then click the Submit button. If needed, you may attach a file, such as a screenshot or log, to the session by clicking the Attach File icon (paperclip), which opens a standard File Upload dialog that allows you to browse to and select the file to attach.