Kon, for some unknown reason, it's called Notes on the display screen, but it's called "Additional Info" when you're in Edit mode. Click the "Additional Info" tab and enter something there.
Calling it 2 different names depending on whether you're viewing it or editing it should be corrected. Really, there shouldn't even be a separate tab. Maybe in SM16, they could just move the Notes field on the main Contact screen.
Kevin