To reach Pueblo Ruins you need to head from ARC Camp past the main headquarters and up the hill. Follow the signs for Laguna Negra.
Once you cross over into Laguna Negra there will be a loading screen. In Laguna Negra follow the main path around the corner and into the town, from the town square follow the path to the right out of town and into Aldea Cliffs.
There will be another loading screen for Aldea Cliffs. From the entrance of Aldea Cliffs head to the left and follow the path, eventually you will see a cave at the end of a valley. Inside the cave you should see the crossover into Pueblo Ruins.
Hope that helps.