Make sure that when you install the Plug-in, that the installer doesn't install the program in a different directory than your VST Plug-ins folder. This is the norm with a lot of installers nowadays, it's up to you to change it to the right path. Try running the installer again and look for the install path screen, then add that path to your Studio One Paths preferences as a place for Studio One to look for the plug-in dll's.