If it where me, I would rack the beer on top of them in the secondary. Since they are dried, I would also boil them to make sure they are nice and sanitary beforehand.
You could put them in to the primary with anywhere from 15-5 minutes left (or the whole time I guess when if you where so inclined) but all of the research I did for a blackberry american wheat I am working on seemed to lean twords putting fruit always in the secondary.
I don't know anything about elderberries and only slightly less then that about brewing so I couldn't comment on if it will taste good or not, but it never hurts to try. Maybe split the batch after the primary, rack half onto the berries and half not if you are either not sure you will like what it does, or not sure you have enough berries to make a difference. Good luck!