cp /etc/xdg/menus/xfce-applications.menu ~/.config/menus/xfce-applications.menu

Then either edit it by hand or use those GUIs.
Note that maybe they're editing /etc/xdg/menus/applications.menu and that isn't used by Xfce so you have to point them to the right file.

Also the desktop-directories and the applications' desktop files themselves interfere on what shows up or not in the menu, so it depends on what you're trying to do.

I have tried editors like menulibre and lxmed, but whatever I do with this applications, I can't make any changes to the menu.

What can I do?

Arch Linux is the distro.