Sambucus nigra is elderberry. The flowers are used in elderflower cordial and I once made "elderflower champagne" which wasn't great, but not poisonously so. I read that unripe berries are mildly poisonous but the ripe ones are traditionally made into wine, cordials and can be added to jams, chutneys and other preserves, from which I deduce that they're not that delicious on their own.
http://www.pfaf.org/database/plants.php?Sambucus+nigra

I think it unlikely that your dog would eat them, but don't let him / her chew the branches as the source quoted above says that the leaves and bark are poisonous.