To get to the first treasure map of the Cranberry Bog region, I fast travel from my camp to the Foundation free fast travel location — pausing, as always, to check whether Minerva has snuck in while I wasn't looking. From there I fast travel to the Watoga Civic Center and head northwest. Look for the river and follow it in that general direction until you reach a road bridge spanning the water. Once at the bridge, duck under it and look for the mushroom growing next to the concrete piling — the treasure mound is right there beside the mushrooms and the piling. Dig.
Run back to the Watoga Train Station to sell your found loot if you're a mind. Don't forget to scrap the swamp mud off your boots and scrap the digestive goo out of your hair before mixing with polite society again. That's Appalachian polite society — so bring some rotting radrat meat with you.