You can add staff as Lunchtime supervisors by adding them to the sitting itself. Sittings already appear in student calendars, so you don't need to set up a separate event.
It isn't possible to add staff to the meals area (such as lunch time supervisors) while setting up your meal sittings. If you click on a meal sitting, you can't add staff.
However, you can assign staff using the School > Timetable > Timetable Administration > Timetable Slots page.
This will add the meal timetable slot to the staff members calendar and help them track which meals they are supervising.
Once you have at least one staff member assigned, it then becomes possible to make additional changes from within the meals setup area.