Make sure the VSTs are 64-bit VST2 (2.4) and/or 64-bit VST3, and are located in any of the following folders:
Under Windows:
C:\Program Files\VSTplugins
C:\Program Files\Steinberg\VSTplugins
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/VST
~/Library/Audio/Plug-Ins/VST
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".
Both platforms:
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.