I donít know anyone who thinks Aperture is the best raw converter, itís easily surpassed in that category by several other converters. What it is, in my opinion, is the best combination converter/editor/cataloger/printer. FWIW.
With that out of the way, how are you trying to install it? Itís not even visible on the App Store at this point, unless youíve already purchased it and had it installed. And if youíre trying to install from a disc, there arenít any versions that were released on disc that would run on anything later than 10.7 or maybe 10.8.