It's surprising that adding an item in the main menu is so difficult.
First, I tried to right click on the main menu to access some kind of editor but there seems not to be any, one has to find and download such an editor from the ubuntu repository.
So, I installed Alacarte and it showed up in the Accessories sub menu.
I then tried to add a menu item, one pointing to the gnome-editor, (EDITED sorry I mean gnome-terminal) but it never shows up in the menu. I tried to add it in the Accessories sub menu. I tried as my user, and later on tried also as root. I logged out and in. Nothing helped.
Also, the Others sub menu does not appear in the menu, even if it is not hidden.
I'm using Lubuntu 12.10