As a further explanation: the reason you can't just put your menu_add commands in init.acl is that init.acl executes before the window is created. Since the main window doesn't exist yet, there's nowhere to put the new menu item when init.acl runs.
Using the menuloadhook solves that problem by making sure the menu_add command(s) run when the menu controls are built. This has the added advantage of making sure your modifications run every time the menu is refreshed (e.g. when a new window is created), and not just when Arbortext Editor starts up the first time.