When native UAD (UADx) plug-ins are installed they are copied to the following default locations:

Windows

VST3 (64-bit)
C:\Program Files\Common Files\VST3

AAX64
C:\Program Files\Common Files\Avid\Audio\Plug-Ins\

Mac

VST
Macintosh HD/Library/Audio/Plug-ins/VST3

Audio Units
Macintosh HD/Library/Audio/Plug-Ins/Components

AAX 64
Macintosh HD/Library/Application Support/Avid/Audio/Plug-Ins 

For default install locations for UAD-2 plug-ins, click here.