Make sure the VSTs are in "VST2" (2.4) format and 64-bit, and are located in any of the following folders:
C:\Program Files\Common Files\Steinberg\VST2
Under Mac OS:
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 Advanced tab - and add the folder(s) where your VSTs are located. Then, restart Reason to have it scan your added folders as well.