Start SO1, In the main menu choose "Studio one" then "Options"
In the new window, select the "Locations" view, that's where you'll tell SO1 where to find the VST"s.
Default location for VST2 executable (dll) is C:\Program files\Vstplugins (or in the directory you specified when you installed the product)
Default location for VST3 executable (vst3) is usually C:\Program files\Common files\VST3 (or > same as above)
Add as much locations you need and check the "Scan at startup" box so SO1 will take those in considerations for next start.
Hope that helps.
And you should check the user manual too for complete info ...