Aye, Nikodemus has it right, but here are some shots to make it easier. This assumes you have the b2cs overlay interface working.
When you start out your screen will look something like this. This is the default b2cs overlay screen. The settings we're looking for are not here though.

Now click on the Sector Settings button as Nikodemus suggested. Follow the red arrow!


Now change the ambient setting from "none" to a value like:
0.3,0.3,0.3 - you need the commas and make sure there are no spaces. While you're at it set the culler to Dynavis. Its probably the one you want. When you are done it should look something like this:

And there you have it! Ambient lighting! You could give it a tint of color if you wanted to create some atmosphere. The values 0.3,0.3,0.3 essentially mean 30% Red, 30% Green, and 30% Blue. So considering that if you wanted a blueish hue to all the ambient lighting just set a value like 0.2,0.2,0.5 for instance. Play with the values until you get something you like.