CS6 cannot open .indd files saved with CC. If you are a CC subscriber, download the CC versions (you get both CC and CS6 as a subscriber).
You can alos download the CC trial and export the file to .idml, which should open in CS6.
CS6 Master Collection is the last perpetual licensed product. The files created with CC version of Adobe apps are not, in many cases, compatible with CS6 versions.
Is there some reason you thought they were?