s230-exposure and long shutter

Hi everybody,
Under what cirumstances should I use more/less exposure (-3 to +3 in the S230) and when should I use the long shutter?
Thanks!

exposure on the s230 seems to always need alittle on the + side. pictures come out dark for me. the long shutter is good for taking pictures in low light (city light at night) and such. what I would like to know is how to make the shutter speed operate without having the s230 automaticly use the light enhancer.