Some plugins have their own installation and activation workarounds. Whilst some
VST plug-ins are only DLL files you have got to copy and paste without running any installation program.
So basically when you are done installing the software you need to scan for new plug-ins in FL Studio by right-clicking on the browser tab and selecting
fast-scan. Your new plugin should now show in FL Studio's plug-in database. By the way, what plug-in are you trying to install?