1. Make sure you've downloaded the correct installer from the Korneff Audio site, for Windows it's the .exe file, for Mac it's the .pkg file, as well as updated to the most recent version of iLOK License Manager.
You can get your iLOK License Manager here.
2. Check if the plug-in is installed to the correct folder for your system.
For Mac, VST3 plug-ins should be in the following folder:
» Macintosh HD:/Library/Audio/Plug-Ins/VST3
AU plug-ins should be in the following folder:
» Macintosh HD:/Library/Audio/Plug-Ins/Components
For Windows, VST3 plug-ins should be in the following folder:
» C:\Program Files\Common Files\VST3
3. Confirm that your plug-in file paths are added to Reason by going to the Edit menu, Preferences>Advanced Settings and making sure that the file paths above are listed under VST Plug-In Folders. If not, add them manually and make sure all needed file path boxes are ticked.
4. In order to rescan plug-ins, restart Reason.