The rotate option isn't available for the org. chart object. One method...
If the org. chart is completed (i.e., you won't need to edit/change it
anymore), you can copy it to the clipboard, then choose Edit - Paste
Special - Picture (GIF) (for example), and the resulting object can be
rotated.
Otherwise, you can change the orientation of any given page to landscape by
enclosing it in its own document section (using section breaks).