It is in the far lower right corner of the map. It will be relieved to you after you do the main quest for Three Dog (Galaxy New Radio) which is the second or third main quest. Or you can walk there. It is outside of the highlighted part of the map (the DC ruins are highlighted and you usually must take a metro to get into them. So you can walk around, or fallow the shore of the river that goes south, between the highlighted areas on the lower right of map. Rivet city is in a huge boat (aircraft carrier) that is just of of shore. There is a bridge that extends to it and the boat is broken in two pieces. You can go to Rivet city before the galaxy news radio quest and skip it all together.