When you edit a visitor by tapping on the visitor detail, you will see the section 'Note' when you scroll down.
Taping on the section underneath 'Note' will bring you to the 'Edit Note'.
Add your notes as required. By tapping on the '< Details' button, the notes are saved and you are back at the visitor detail.
Note: Depending on the capability of your device, you will see a microphone in the keyboard to record your message.