Night Scene mode might work, but you're probably going to need to put it in Complete manual (Where you have control of both the shutter and the aperature), or Tv or Av - Shutter or Aperature adjustment only, respectivly.
I'd go with manual. If you don't have a tripod, you can use the 2 second timer.. Set the camera up, press the shutter and let go.
Anyway, you're going to want the most seconds possible, and the lowest f/ number possible. Make sure the camera isn't zoomed.
Try at something like 5 seconds first... Remember not to touch the camera until it is done.