Jimmy Carter Presidential Library, 441 Freedom Parkway, Atlanta, GA 30307. Park for free in the western section of the library campus and walk to the enclosing wall (above Freedom Parkway) that faces downtown Atlanta. You will have arrived when you can see the skyline over the wall.
Turning in from Freedom Parkway, park in the western section of the Carter Center parking lot. The best area is marked in blue on the map below (click for a larger version). Parking is free.
Walk up the hill and the western edge of the lot (pictured below), toward the traffic circle in front of the Cecil B. Day Chapel (the red pinpoint on the map).
At the top of the hill, you'll be able to overlook Freedom Parkway and see the Atlanta skyline.