Turner Broadcasting has two major locations in Atlanta.  One is at 1050 Techwood Drive near the intersection with 10th Street (see map below), walking distance from the Midtown MARTA station on the red/gold rail line.  Take the 10th Street exit out of the station and turn left (turn right if exiting the station via the Federal Reserve exit) on 10th Street.  Walk three blocks then turn right onto Techwood Drive.  The walk is less than 10 minutes.

The other location is at the CNN Ceneter in downtown Atlanta.  Exit at the Dome/GWCC/Philips Arena/CNN station on the blue/green rail line and walk about five minutes down Centennial Park Drive.

For more information, see:
MARTA to CNN
MARTA Accessible Companies
Navigating the Midtown Station
Navigating the Dome/Philip’s Arena Station
Using Google Maps for MARTA Directions