PhotoShop Preferences / Additional Plug-Ins gives this path: Mcintosh HD: Library: Application Support: Adobe: Plug-Ins: CS6: File Formats.
But I can't access this library in any of the ways suggested: neither Option-clicking on Go nor ~/Library take me to that library.
Where is it?
You want the global /Library folder, not the user ~/Library folder. Leave out the ~
Retrieving data ...