[SixOfSpades]

Getting your map updated to get into BG isn't necessary--the city is already on your map when you start the game. Look at your journal--if it says 'Chapter Five,' then (unless you have a bug) you can enter BG. Go north from the Friendly Arm Inn, that's the Ankheg map. Go north again, and that's the Bridge map (Wyrm's Crossing). You should be able to get here no matter WHERE in the game you are--you could go to this map without even clearing the Nashkel Mines, but you won't be able to go ACROSS the bridge until you've beaten the Cloakwood. Now, the tricky part most people have is this: Once you're across the bridge, go to the western edge of the map, and you'll see the outer wall of BG City. You have to kind of hug this wall and walk along it until you clear enough Fog of War to see some walkable ground INSIDE the wall. Go under the archway that you can't see, then go West onto the new map. Ta-da, you're in the city.
Go North from Wyrm's Crossing to go to the Zombie Farm, or Northeast to go to Ulgoth's Beard (if you have ToSC).
Now, if your Journal says you're in Chapter Five and you STILL can't get to BG or the Bridge map, you have a bug. Follow the steps I outlined and if you still can't get in, let us know and we'll help cheat you there. (We are very familiar with using the cheat codes to get around the bugs in the game.)