Jamsil Stadium is conveniently accessed from Line 2 - Sports Complex Station (종합운동장역). The Doosan Bears' website has excellent information on getting to the stadium.
The Doosan Bears' Website is an excellent source of information in English.
The Doosan Bears and LG Twins share Jamsil Stadium as their home stadium. Check the schedule, and be sure to catch a game between these two teams. The game atmosphere is always a little more electric during these games.
Doosan Bears Twitter Feed
Websites on Baseball in Korea: