Hey i had the same damn problem pissing me off all day and no one seemed to be able to answer me or they think i'm an idiot for not figuring it out myself anyway this is what i did.
I followed the instructions at http://wiki.xfce.org/faq#menu
but all that did was show the other menu items in the menu editor but they were attached to empty duplicate folders in the menu and i still couldn't edit the ones that were actually being used so i ditched that method.
The default .xml that the menu editor seemed to be loading was /home/possum/.config/xfce4/desktop/menu.xml so what i did was rename the original menu.xml to menu.xml.original and replaced it with the /home/possum/.cache/xfce4/desktop/menu-cache--home-possum-.config-xfce4-desktop-menu.xml.xml which i had seen in the xfce wiki instructions and renamed it menu.xml.
So i can now edit my menu items in the menu editor but for some reason when i install new applications the shortcuts dont automatically appear in the menu and i have no idea why.
I spent a whole day trying to figure this out and its such a damn simple concept to edit a damn menu . I'm not an idiot i just very new to linux and having trouble learning some things. If anyone can please tell me how to do this easier and why it behaves this way it would be much appreciated.