ARA2 is not supported by FL Studio: ARA2 Plugin
However you can assign SpectraLayers as an editor into FL Studio settings: Image-Line FL Studio
If you did not install SpectraLayers in C:\Program Files\Steinberg\SpectraLayers 7.0, just replace that part with the folder where you installed SpectraLayers: [MySpectraLayersInstallFolder]\Win64\SpectraLayers.exe