That may be the problem, that perhaps you never installed the plugin when running the installer. You can see where the installer may have by default installed the plugin files by running the installer up to that point just to see what default path is, and checking that path to see if the plugin dlll is there, and if so cancelling the installer.
Assuming that the plugin is installed on your system, the checklist below would be everything to check to try to troubleshoot why some plugins aren't showing up in Studio One …
It could be that Studio One isn't scanning it. Visually look for the plugin dll in the folder it was installed in to make sure that it's where you think it is, and if so, check Options > Locations > VST Plugins to make sure that path is listed there, that Studio One is also scanning plugins from that folder.
Also, make sure it's a 64-bit plugin dll if you’re running Studio One x64 and likewise if running Studio One x86.
Additionally, it may have been blacklisted so clear the blacklist from Options > Locations > VST Plugins > Reset Blacklist and restart Studio One. You can view the startup messages after the fact from menu View > Additional Views > Messages to see if there are any warnings or notices there about the plugin.
Lastly, check that it's not just hidden. Click the wrench icon in the browser to go into that mode, where hidden plugins have the white circles dimmed.
Also, on Windows if it's an x64 VST3 it has to be in \Program Files\Common Files\VST3 to scan and load.
If the issue remains, please create a ticket to technical support.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new