First try to see if the plugin is installed, something like
C:\xxx\Petrel 2012\PluginManager.exe /list C:\xxx\Petrel 2012\Petrel.exe
If the plugin doesn't list, try
C:\xxx\Petrel 2012\PluginManager.exe /install /allusers thepip.pip C:\xxx\Petrel 2012\Petrel.exe