I am assuming that you are stating that the path to VST plugins path(s) are missing in the Studio One options. Without this Studio One will not add the plugins for use in a song or project.
VST plugins have default locations depending on Mac or PC. It is also possible to install plugins to multiple or a different path than the default. Knowing this here are the some default locations:
C:/Program Files/VstPlugins/ (for 64-bit plugins) C:/Program Files (x86)/VstPlugins/ (for 32-bit plugins)
The location can be changed at any time. For that it would be wise to google that process for your operating system. If you do not install for all users it could also change/add from the default path. This is only possible in some installers but should be noted.