FLStudio on macOS is similar to using the plugin on Windows.
Install and register Nexus 2 on your Mac per the instructions provided by reFX.
Once installed, in the FLStudio Browser right-click the 'Plugin Database' folder and select ...
Frankly speaking, if I knew that the simple VST installation could be so full of bugs, I would have thought twice.
Regards
Eugene
P.S.
As an afterthought: I obviously, do not have FL installed, nor any other Image-Line software. ...