What have you installed that is version 8.3?
Camera Raw 8.3 is not compatible with CS5. You can go to PS-CS5 / Help / About Plug-ins / Camera Raw and see what version you have.
If you mean you have installed the DNG Converter 8.3, then installing it isn’t’ enough, you have to run it and convert your raw files to DNGs so they will open in an older version of the ACR plug-in.
How do you run it and convert the files after you have installed the DNG
It is a standalone program. Start the DNG converter and point it to a FOLDER containing your raw files. Don't open the folder, just point the DNG converter to the folder. Then save your other options appropriately and click on the Convert button.