On February 7, 2008, about 1815 eastern standard time, a Cessna 172RG, N6562V, registered to and operated by Aviation Atlanta Inc., had the right main landing gear collapse during landing at the DeKalb-Peachtree Airport (PDK) in Chamblee, Georgia. The instructional flight was operated under the provisions of 14 Code of Federal Regulations Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The certified flight instructor (CFI) and pilot rated student reported no injuries. The airplane sustained substantial damage. The flight departed from PDK on a local training flight on February 7, 2008, about 1530. According to the CFI, he and the pilot rated student were approaching to land on runway 20R. After completing the pre-landing checklist they checked the under-wing mirror and noticed that the main landing gear were not down and locked. They made every attempt to lower the gear but were unsuccessful. During landing the right main landing gear collapsed buckling the right elevator. Both occupants egressed the airplane without injury. Examination of the landing gear by an FAA Inspector found the nose gear Actuator O-ring cracked and leaking hydraulic fluid and all fluid had been exhausted..