How to install?
Simple copy the extracted DLL  to %USERPROFILE%\Documents\Fiddler2\Scripts\. When you start Fiddler2 you will see the plugin running in the Fiddler Options -> Extensions tab, it will identify itself as MS.Tools.PrettyPrint.
Which response are reformatted?
If the plugin is reformatting the response it will add a custom http header X-Fiddler-Plugin with PrettyPrint as value.