What is an elderberry and how do I find it thanks

There's no real way to tell exactly where all the different herbalism plants are in granite falls, I think they spawn differently each time. Basically, just pause the game and scroll all over the place, mousing over any garden-plant looking things. Anything "unidentified" is one of the new herbalism plants (http://www.carls-sims-4-guide.com/skills/outdoorretreat/herbalism/), you just have to harvest all you can find (once they're ready to be harvested, some may not yet be ready) and have your sim work on identifying them. Even once you've identified them, the plants in granite falls will still show as "unidentified," so it can be difficult to find a particular plant again, but once you harvest an already-identified plant, it'll show up identified in your inventory (I had a sim identify all the unidentified plants, but all the herbalism plants in GF still show unidentified, but they show up properly in her inventory when she harvests while camping). I'd definitely recommend that, if your sim is serious about herbalism and brewing remedies, you create a garden of the herbalism plants on your home lot, that way you can have a never-ending supply. (Unfortunately, the bugs you need for some remedies can only be caught in GF, but at least you're most of the way there with growing your own plants.)

Good luck!