Quoth my girlfriend, the Baltimore city native:
"
1. Walk out of the train station and turn left. That is St. Paul St heading downtown. All the way at the street side is a bus stop on the left. You take the #3 bus. It costs something like $1.60. It dumps you just about right next to the Convention Center.
2. Find out which track is the Light Rail. Take the Light Rail to the Convention Center stop. Hard to remember right? You will see the HUGE Hilton beast on your left and Camden Yards baseball stadium off to your right. It is a small stop, you get let off next to a parking lot. If you go one stop further, no worries, you just have a bit more walking to do. The fare is still $1.60.
http://www.mtamaryland.com/services/lightrail/schedule/
Hope that helps. The #3 is fairly reliable for a Baltimore bus, the Light Rail is supposed to run every 15 minutes-ish. The schedule will tell you more. And you’ll want the “southbound” one. "