Hopefully you have found a solution to this but here is what has worked for me, when working with third-party VSTs (on a PC) in Studio One:
Go to the 'File Explorer' on your computer and create a new folder in 'Documents.' Name this folder after the company that made the VST (ie, "Neural DSP"). Now inside this folder place the file for the VST, such as the ".dll" file.
Start Studio One, and go to 'Locations', (this is the same menu where 'External Devices' and audio setup are) and 'add' the location in 'Documents' where the VST is now located. Make sure the path is correct, and delete other paths where Studio One cannot find the plugin (ie, 'Program Files'; 'Program Files x86').
Tell Studio One to reset the 'backlist' from the same menu. It should rescan for the new plugin at startup.
Go to the 'Plugin-Manager' and reset the blacklist also.
Close Studio One, then reopen it. You should see it scanning for new plugins while it is loading.
Go to the 'Plugin-Manager' and see if the new plugin is there. If not, 'Update Plugins' and see if it appears.
I use Pro Tools, and it automatically scans and finds every plugin on the computer immediately , this is just something that Presonus lags behind the rest of the industry in.
I hope that helps.
Music 7 Studios