You have to add it like an application. See http://douwil7.100webspace.net/linux/Tuning.html#4 . There should be a shutdown desktop file in /usr/share/applications. Be sure it is correct to display a shutdown in one of the menu categories from which you can add it to the taskbar panel in the usual way: Exec=lxde-logout NoDisplay=false OnlyShowIn=LXDE; Categories= Put in Categories where you want it to appear in the main menu, or leave it blank to appear in Others.
thank you (for both your answers on this topic and the other one).
It's a bit complex for me : I'm one of these new linux users, quite similar to most mac users I believe...Using only linux at home for the 6 or 7 lasts years, but still never use a terminal or notepad if I'm not forced to