If you design for landscape mode, like a game or a video player, you can use Rotato's native landscape feature.
Drag in your landscape mode design file. Make sure you've selected a device that supports landscape mode in the real world - i.e. not a laptop. Rotato will now automatically fit the design and rotate the device.
If you want your mockup to show your landscape mode design in portrait mode, you can manually override the rotation. Go to the Looks tab and click the portrait mode button right below the scene selector.