Bus 500, operated by Clayton County Transit , leaves from curbside at the North Terminal and makes a stop right outside the main entrance to Delta HQ. Travel time is only like 5-10 minutes, check the website for schedules.

Perhaps an easier option is to take the hotel shuttle for either the Renaissance or the Hilton airport hotel. There is an unwritten agreement that these shuttles will drop you off right outside Delta if you tip them a buck or two.