Forms!YourMainFormName.YourSubformCONTAINER.Form.AllowEdits = True
Where YourSubformCONTAINER is the name of the control on the main form that houses the subform. If it is the same name as the subform that is okay, but if it is not you need to use the container name and not the subform name.
Also, you need the .Form. part in there to tell Access you are wanting a property or method of the subform itself and not of the subform container.