Atlanta CSS 2015 will be held near Georgia State University at the Loudermilk Center.
You can see a map of the area here
If you plan on using a car to get to the venue, there are several parking options. There is a parking garage directly west of the Loudermilk Center that will charge an all-day rate of $8, but you must get your parking ticket validated inside by Loudermilk staff. Additionally, there are several parking lots to the north and east of Loudermilk that offer all-day rates ranging from $5-7. These lots are located at the intersection of John Wesley Dobbs Ave NE and Courtland St. NE.
Keep in mind there are options for public transportation downtown as well, including MARTA and the Atlanta Streetcar. The nearest MARTA stop to the Loudermilk Center is Peachtree Center (on the North-South line), and the Atlanta Streetcar stops right outside the Loudermilk Center at Hurt Park.