Try this:
frmMasterlistAdministrationAdd frmMasterlistAdministrationAdd = new frmMasterlistAdministrationAdd();
frmMasterlistAdministrationAdd.TopLevel = false;
frmMain frmMain = new frmMain();
frmMasterlistAdministrationAdd.Parent = frmMain;
splitContainer3.Panel2.Controles.add(frmMasterlistAdministrationAdd);
frmMasterlistAdministrationAdd.Show();