By default, Reason 9.5/10 and Reason Essentials 9.5/10 look for installed VST (2.4, 64-bit) plugins in the following locations:
C:\Program Files\Common Files\Steinberg\VST2
Under Mac OS:
It’s also possible to add your own custom locations to the “search list” if you like. You can do that on the Advanced tab in Preferences.