I ran into this issue as well with Adobe Acrobat. I would imagine this should work for all plugins though.
To install plug-ins into LoadRunner (Example is with Adobe Acrobat):
Install Acrobat Reader on Firefox on the controller/scripting machine.
Find the npppf32.dll file in Firefox’s folder on controller/scripting machine
Copy the file to %LR_Path%\bin\firefox\plugins
*if plugins folder does not exist, create a new folder called plugins
Copy the file to %LR_Path%\bin\gecko\plugins
*if plugins folder does not exist, create a new folder called plugins
