The Editor applet is disabled when a browser on the Macintosh is detected. If a Rich Text field is set to display "Using Java Applet", and a Macintosh browser is detected, a text input field will display using HTML instead. This is due to design limitations in the Java implementation for these browsers.
See the "Minimum browser support for Domino Java Applets" document in the Things you need to know chapter for baseline Web browser requirements.