Light orientation works in degrees, just like the camera. 360 degrees of vertical and horizontal rotation (plus roll for the camera, the 3rd rotation input). The default fill light setup I provided has lights at 0, 120 and 240 degrees. Look at your camera's horizontal rotation (the middle rotation value) and you should be able to extrapolate where the fill lights are in relation. They are at 45 degrees vertical, halfway up from the horizon, by default.
This is also all based around the 0,0 coordinate origin. The further you get from the origin the less intuitive the rotation figures will be because they are referenced to 0,0. But in any case assuming you're near 0,0 (within a few 10's of kilometers) it shouldn't be hard to guage.
You may find it best just to move one of the fill lights to the right position, perhaps just off the orientation of your camera. But remember this is the direction it will be shining *from* - if you want the back (camera-facing) side of a tree on the right side of your image to be lit up, you'll actually want the fill light *behind* you.
- Oshyan
