textrain - You need to click on one of the 'knobs' surrounding the clock to set the time for the various environmental elements to take effect.
That will then give you access to the controls/ sliders for each of these settings, including fog density.
Click on an item (like the Sunlight or Ambient lighting panes) to activate the relevant controls for that element.
You can also use your mouse to add additional time-points (knobs) to the 24 hour clock to fine-tune the time that such fog or lighting effects take effect by clicking on the Plus icon on the left and then inserting a new control point onto the concentric ring around the clock.