FLStudio includes a wide range of plugins: generators, tools and effects. However most of these are FLStudio format only. Which means they only work in FLStudio.
If they are available as a VST or AU format ...
FLStudio will look for the FLStudio engine. Normally this is the location of FLStudio that was most recently installed. You may want to use another installation of FLStudio, for example an Alpha or Beta for testing ...