Your Builder automatically comes with a mobile friendly website. To access your mobile site settings, first click on the Builder icon.
From the Builder control panel, hover over design and click on mobile.
The first option you'll see is Active. To turn your mobile site on, click on the Yes radio button. To turn your mobile site off, click on the No radio button.
If you've made any changes here, make sure to click on save changes to apply them.
Note: As of April 2015, Google gives preference to mobile friendly websites on mobile searches. For more information about this, click here.