So I installed Spectralayers from the Download Assistant and that installs Spectralayers to the default location being C:/Program Files… However I wanted to have it in D: so I installed it manually. Then the ARA plugin didn’t show up in Audio/Extensions. of Cubase
I copied the Spectralayers.vst3 file found in the ARA/VST3 folder where you have Spectralayers to the Common Files/VST3 folder where all VST3 plugins reside and then started Cubase and lo and behold the ARA extension was recognized and functions without problems.
Hint to the developers/packagers. Add the option where to install the VST3 during the setup procedure and locate the default VST3 folder by searching the registry key of Cubase for that path.