Dogmeat is found in Scrapyard. On the world map, Scrapyard is in the top right corner of the square that is two squares directly below Minefield. Follow the clear triangle on your compass if you are having problems finding it. Go into Scrapyard and look/listen for fighting. Your will find Dogmeat fighting a Raider. Once the Raider is dead you can talk to Dogmeat and ask him to join you.