C:\Program Files\Autodesk Revit MEP 2010\Program (64 bit and 32 bit OS)
C:\Program Files (x86)\Autodesk Revit MEP 2010\Program (32 bit Revit installed on 64 bit OS)
If you double click the file to open it, most likely, the VSTA (Visual Studio Tools for Applications) interface that is part of Revit will open to let you edit it. I created a video that clocks in just under five minutes to show the file and what I did to change the report. Here's what the finished report looks like.
Here's what the html file looks like after I added some text to the Description parameter field that I added.
You can just listen and watch the video here now too.
I've written about Panel Reports before HERE and HERE.