Make sure the VSTs are 64-bit VST2 (2.4) and/or 64-bit VST3, and are located in any of the following folders:
C:\Program Files\Common Files\Steinberg\VST2
C:\Program Files\Common Files\VST3\ (VST3 plugins if you are running Reason 12.5 or later)
Under Mac OS:
Library/Audio/Plug-Ins/VST3 (VST3 plugins if you are running Reason 12.5 or later)
The user (~) Library folder is hidden by default. To open it, press the Option key while opening the "Go" menu in Finder, then select "Library".
If your VSTs are in other folders, open Reason's Preferences - the Folders tab - and add the folder(s) where your VSTs are located. Then, restart Reason to have it scan your added folders as well.