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 plug-in file paths inside Reaper by going to Options>Preferences>VST
If the above file paths are not listed under VST plug-in paths, please add them there, separated by semicolons in case you have multiple paths.
Rescan plug-ins by clicking the Re-scan button, make sure the Scan new/updated plug-ins on startup box is ticked and restart Reaper so it can scan your plug-ins.
Note: on Mac, Reaper scans AU plug-ins automatically at startup.