I am not sure if it will work, but doing this should open the first SubMenu of the main menu:
CMenu* pPopup = mainmenu.GetSubMenu(0);
pPopup->TrackPopupMenu( TPM_LEFTALIGN, ptMousePos.x, ptMousePos.y, this );
Obviously, you would have to specify the coordinates where the SubMenu should open.
If you can compile your program with the latest MFC (with the feature pack) you could switch to CMFCPopupMenu
]. It is much nicer, with an optional animation and has more methods