From what I have read from time to time, I would approach it like you have a medical condition that needs accommodation. That you will need X amount of time every X hours to pump for your baby, a private place to pump that has an electrical outlet, and a place to store your pumped milk. It would also help if you had a note from your doctor that details what would happen to you medically if you don't pump (painful engorgement, mastitis) and that you need to pump regularly to adequately feed your baby.
Pumping for such a young babe isn't really compatible with jury duty; they may just let you go. Working moms are generally not given as much leeway since you are already out of the home, but if your work place accommodates your pumping then they may see it as a reasonable request.