From what I can tell this is issue with the plugin installer itself, not with Fiddler, so I suppose you should try to contact the developer of the plugin (Microsoft I suppose) and ask him for help.
However, I managed to workaround the problem and I will provide you with the steps to install the plugin in the latest version of Fiddler. Please, keep in mind that this is not how it is supposed to be done normally and you are doing it at your own risk.
First download the .msi installer. Then unzip it (yes, it can be unzipped, I used 7-ZIP). Inside you will find two files - "Manual" and "neXpertDLL". The actual plugin is the latter one, just rename it and add . (dot) before "DLL". Then copy this file to %LOCALAPPDATA%\Programs\Fiddler\Scripts. Start Fiddler and the plugin should be loaded appropriately. The "Manual" file is actually html, so you can rename it and add the ".html" extension. This seems to be manual how to use the plugin.