I'm using Chrome on my Android phone (LG Volt), and I get that option when I open ATP, hit the lower right menu button. One option is "request desktop site". On my Kindle Fire HDX tablet, using the Silk browser, I open ATP, use the menu button on the right, and I get options for "request another view". If I choose that I can then pick "auto" or "desktop" or "mobile"
Poor Dave, I'm waiting for someone to ask him to create an ATP mobile app. One for Apple, another for Android...
Karen