Q: I am trying to replace ACR 3.0 by 3.1 (or higher) I cannot find ACR in my Photoshop/Plug-in/File Formats folder, where is it located now?
A: As explained in the "Readme", which can be found on the ACR download page, the new correct location is:
Mac: /Library/Application Support/Adobe/Plug-ins/CS2/File Formats/
Win: \Program Files\Common Files\Adobe\Plug-ins\CS2\File Formats\
Mac: /Library/Application Support/Adobe/Plug-ins/CS3/File Formats/
Win: \Program Files\Common Files\Adobe\Plug-ins\CS3\File Formats\