"Av" - is aperture priority, meaning you just change the aperture manually and the camera works out the correct shutter speed.  So all you need to do here is use the lens "wide open" meaning set it to the lowest number shown by the "Aperture" in the very useful diagram above... check that the shutter speed is ok and not something like 1/30th or slower (1/15th for example).  If it is then you need to put the ISO up a bit more (if you can).

AI Servo - is the autofocus setting which means that the focus will track moving objects... to be honest - you are probably better using manual focus in these lighting conditions as your camera will struggle to focus automatically.  Practice a bit beforehand and you should be ok... i know with a fisheye it is just a case of setting the distance generally and then not bothering with focusing ;)

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography