Is it in the "Start Game" Area?
No, you must be the server HOST (The person to make the server), not to be confused with SERVER ADMIN or a normal ADMIN, you need to hit F11 for Mission Editor or F10 for the GUI editor
I advise you people not mess with the GUI editor unless you know what you're doing, otherwise you may break something (Or not?), with the Mission Editor, the only thing you'll manage break is
GlaDOS' heartyour server