Saving to Text file, and calling a method!

I have created an application which is a calendar which Ill call C. This C is currently using a MainForm which Ill call MF and Appointment class which Ill call AC and Appointments class which Ill call ASC which inherits List<IAppointment>, IAppointments.

Mainform - MF, is adding appointment to Appointments List

IAppointments _Appointments; - appointments are been added into this from AC constructor

_Appointment = new Appointment(NA.GetSubject, NA.GetLocation, NA.GetLength);
                _Appointments.Add(_Appointment);

This is all good and well but what am trying to do is save the appointments to a text file which is been done in a method in Appointments called Save.

So my first thought would be to get _Appointments which I would do by MainForm MF = new MainForm();

MF._Appointments, but its not there.

What am I doing wrong!!!