How are directional lights supposed to work?

[UpDog]

I've been trying to get some sort of light effect, anything, for most of the evening now. They aren't lighting anything at all.

How do people use directional lights, and what scenarios would cause them to appear to do nothing?

The environments I've tried are :
a medium sized room, with a number of pillars, directional light in center and top of room.
a hallway with a duct/grate in it (grate being a static mesh) and a directional light and dominant directional light behind the grate.

Both render completely dark after build.

[WolverineFRS]

The directional light has no position, no matter where it is in the level. It has only direction and an infinite long ray length. It's useful for outdoor scenes to simulate the sun. I'd recommend you to use point and spot lights in your indoor scene.

[Star Weaver]

What causes directional lights to do nothing is there being something between the place not getting lit and the edge of the universe that the light's supposed to be coming from. So, if you're indoors, it's going to be your ceiling. You could cut a skylight in it, thought. If you're outdoors and it's not working, check your skydome mesh and make sure all of its lighting options are disabled, but particuarly the shadowcasting ones.

[UpDog]

Very helpful, thanks all!

[Star Weaver]

(BTW, pretty much every time I pick up UDK after not using it a bit, I immediately plop down a light-blocking skydome and spend half an hour going WTH about it )