Where can I find the installed XLN Audio AU, VST or AAX files?

You can find your plugins at the following locations:

OSX

AU: [user]/Macintosh HD/Library/Audio/Plug-Ins/Components

VST: [user]/Macintosh HD/Library/Audio/Plug-Ins/VST

AAX: [user]/Macintosh HD/Library/Application Support/Avid/Audio/Plug-Ins/

WIN

VST: (specified by user at time of installation)**

By Default:

C:\Program Files\Steinberg\VstPlugins

C:\Program Files\Common Files\VST3