Verify scans ensure only the correct plugin types are shown at the various 'add plugin' locations in FL Studio. This correctly sorts plugins into their Installed > Effects or Generators category. Only use the Plugin Manager to add additional plugin install and search locations if you have a specific and valid reason. NOTE: The best manual method is always to install to one of the default plugin locations for your OS. Watch tutorial videos here How to Install VST / AU Plugins (Instruments & Effects)
This page provides step by step instruction on installing VST/AU instruments and effects.