The WellStar Atlanta Medical Center is at 303 Parkway Drive, which is about a 20 minute walk from the Civic Center MARTA station on the red/gold rail line (see walking/area map). It is a straight walk down Ivan Allen Jr./Ralph McGill Blvd. After exiting the station, head south on West Peachtree Street toward downtown and turn left on Ivan Allen Jr.
You can also connect with bus 16, which departs from Five Points and travels along Ralph McGill with a stop at Parkway Drive and Boulevard.
For real-time bus locations and schedules, use the MARTA on the Go app or look up schedules online.