To make sure all pages use this backend layout, edit the page properties of the root page by right clicking on the globe icon and choosing "Edit" or clicking on the edit-page icon in the editing window:
Click on the "appearance" tab and select the "Main 1 column with menu" backend layout for both this page and its sub pages:
And save your changes.
Please note for TYPO3 6.x: since you have set a general storage folder for your root page (and you have if you use TYPO3 6.x, because this tutorial says you need to do that), you MUST place your backend layouts in the storage folder. If you place them on the root page, they will only be available on the root page in this case!