For VST 2.x, you have to create a folder to put the .dll’s and point to it via Plugin Info-VST2.x Paths and re-start Cubase. You can point to this folder for the installs, also. As installs usually want to put their .dll’s in a folder called VSTPlugins in the main Steinberg programs folder, that a good place to do it. Don’t put it in the Cubase programs folder. Cubase’s bit bridge has issues with some VST 2.x plugs. You can see if their listed in the blacklist.XML file in Prefs and if so delete the file and restart Cubase. Many use J-Bridge to avoid Steiny’s bit bridge altogether.
It’s a file in your Steinberg App Data folder (see Prefs Files link in sig) that contains any plugs Cubase had an issue with. Sometimes deleting this file and re-starting Cubase solves the issue of a plug not showing.