The default library has always been: C:\Program Files\vstplugins
If you're using a different directory, you'll need to add it to your list of locations for VST Plugins in Studio One Options.
Click on Studio One Menu, click on Options, (shortcut is control + comma), then select Locations icon and VST Plug-ins Tab.
Click the "Add" button to navigate to the folder you have your plugin installed in and click on Select Folder to add that new directory.