When you are making a card, there's the frame which says "Auto". Change it to "Split/Flip/Double-Faced". An amusing dancing list will show up saying "Normal". Choose "Double-Faced" there, and you'll automatically have the sun and moon faces. (Remember to put a color indicator in the moon face.)

For what it's worth, I'll be changing this interface a little soon. But the label for the drop-down will still be "Frame". That'll be where you force something to use (or not use) the planeswalker or token frames, as well as how you select DFC, split card, flip card, level up, plane, scheme or vanguard. (I might not get every one of those options into the first release.)

Thank you :)