There are two Apple stores in Atlanta near MARTA: in Lenox Square Mall and Perimeter Mall.
Lenox is closer to downtown Atlanta and accessible via the red or gold rail line (see MARTA directions to Lenox Square Mall). Perimeter Mall is further north and off the red line, next to the Dunwoody MARTA station.
Both are easy rail commutes with minimal walking time.
Malls and Stores Near MARTA
Using Google Maps for MARTA Directions