something i have wanted to do for a long time capturing lightning but never got round to trying but i would aproach the subject pretty much the same way i capture firework displays
i would set the camera up manualy with apeture around f8 - f11 something i would experiment with considering iv'e never done it before
iso at 100 ( as allready mentioned ) and shutter on bulb
camera on a tripod with remote shutter release
point the camera where the lightening has been striking and hold a black piece of card over the end of the lens , open the shutter and pull the card away from the lens when the lightening strikes or when you think it's going to strike
do this a few times to build the picture up by holding the black card infront and removing it when you think the timings right
hopefully this should help with the overall exposure and with a little luck a few strikes of lighening
like i said i havn't tried this approacch yet but it's the way i would go about it