When you click on Add a planet, it will appear right there in the view port, in an unobscured area. It will be a bit further than the sun so it can be illuminated by the sun so you can see it. If the sun is behind the planet, you won't see the planet because nothing shine on the face you're looking at, hence black, no planet.
If you move the Sun, it will also change how the planet is illuminated and it may not illuminate it at all or just a bit (like a crescent).
The Sun/Moon position is simple, Elevation of zero means sunset/sunrise position. Azimuth is to rotate the sun around the camera.