Access the case as if to add a new note, then to the bottom of the screen, click Edit
Scroll to Case Folders - Click to Add, Rename or Delete (depending on your Access Control)
Click Add, then give the folder a name, click Save
Scroll to the bottom of the screen and click Save
Reopen the Case Note - the folder will now be there to save notes to.
Should you find any problems with this please log an enquiry with screenshots.