Plug-ins (AU and VST/VSTi) can be placed inside any folder.
On Microsoft Windows, default VST Plug-ins folder is generally C:Program Files/VSTPlugIns.
On Macintosh, plug-ins are located at /Library/Audio/Plug-Ins/.
In the setup / Plug-ins Configuration tab you can specify up to 4 different folders:
If you add or install a new Plug-in while Usine is running, you can update the Plug-in folder without restarting the program with the rescan changes button.