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 ...
FL, I use the manage plugins menu.
When I click the find plugins button, it does a quick scan and finds my plugins.
I click the check mark next to the plugin I want to add, and then close the window ...