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. Add plug-in paths in Studio One by going to the Preferences>Options>Locations>VST Plug-Ins and confirm that the file paths above are listed. If not, choose the Add option and enter the file path.
4. Rescan plug-ins by clicking the Reset Blocklist button and making sure that the Scan at Startup box right above it is ticked. You can now restart Studio One so that it can rescan your plug-ins.