You are creating a new instance of frmMain in your keydown handler. This
fam.addStaffDetailsToolStripMenuItem1.Selected
will always be the same value that is the default setting during design time when you instantiate a brand new form object.
Personally I would add a parameterized constructor into the child form frmStaff and pass in a value to determine the mode of the form. e.g.
frmStaff frmS = new frmStaff("addMode");
If you don't want to do that you will need to try and get the parent form and do a type check to see if it is frmMain then cast it as frmMain to access the child controls that are either internal or public accessible.