A couple of things help, first make sure you have turned on both the sun and the aura (click in the checkboxes). Then to help locate the sun in the preview box, make the aura very large. By making the aura very large, it makes it easier to determine when the sun is getting close. Then move the sun around but keep it close to the horizon until you can see the aura. Once you see the aura, then it becomes easier to actually get the sun into the preview screen (and thus into the render). This technique also works well for finding the moon in a night sky.