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 the 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. Rescan plug-ins in Cubase by going to Devices>Plug-in Manager. Click the cog wheel button at the bottom of the Plug-In Manager window to open the VST Plug-in Path Settings, then click the two arrows in a circle symbol (Rescan All).
If your plug-in is still not appearing, make sure that the above paths are set in Cubase's VST Plug-in Path Settings. If not, click the + sign and add them to the list, then click OK and move on to the Rescan All symbol mentioned above.