In your MainFrm.cpp class, try this:
CMenu* pMenu = GetMenu();
m_bAutoMenuEnable=FALSE;
CMenu* subMenu = pMenu->GetSubMenu(2); //3rd menu item
// disable 1st submenu item of the 3rd menu item
subMenu->EnableMenuItem(0, MF_GRAYED | MF_BYPOSITION);
// enable 1st submenu item of the 3rd menu item
subMenu->EnableMenuItem(0, MF_ENABLED | MF_BYPOSITION);
DrawMenuBar();