I see in 5.2 that the CGNS option appears as “CGNS Files (Plugin) (.cgns)”. The “Plugin” part does not mean you need to obtain and load some plugin. I’m not actually sure why it says that. Did you try loading a CGNS file with it? By the way, you shouldn’t even need to specify the reader type when opening a CGNS file.
As for the version you need to use, until you tell us where it came from (whether it was built by a system administrator, for example) we cannot help you.