To be fair, MK, he was asking for help with a pre-existing engine. Malonfan, feel free to ask the author of that engine for any assistance you need.
Oh !@#$%, that's me!
Well it's actually pretty simple, just look in the Create event of the clock object. There are some global variables there, one of them turns on the day/night lighting. Also check the Game Information Text, there's tons more info like that.