I have posting quite a bit about potty training since this is where we are at with Nichole right now. She is getting it, and I know that eventually she will be potty trained. It might take us a couple months, or a couple of years, and that is okay.
I do know that day time potty training will be easier than night time potty training. I also know of a few parents of older children with Down syndrome that have mastered the day time but not the night time.
Since I am no expert, and it would be silly for me to give advice in an area we have not yet experienced, I will do something better. Go read my friend Leah's blog on night-time potty training and a great little tool that has helped both her children with Down syndrome.