To set the light:

1. Go into the edit mode
2. Select the faces which will reflect the light (if you select more than one the add-on will automatically average selection).
3. Place the light using the "Place Light" button.

After you have set the light you can exit the edit mode and modify the settings anytime later with the "Modify Light" button.
To use the "modify light" button:

1. Select the light you want to modify (must be created by the addon)
2. Press the modify button.

If you have any questions or problems please contact me on here on blender market.