Make sure that you have Nuendo 10.1.0.236 installed, and SpectraLayers ARA VST plugin is checked during SpectraLayers install (it should be by default).
Then, you should find the Extension menu in 3 different places :
-At the top of the timeline, right side, but your screen needs to be at least 1920x1080 so it appears there
-In the Audio menu
-by CTRL+right-clicking an audio region
If you still don’t see it after checking Nuendo and SpectraLayers install and those 3 different places, make sure you can see a file called SpectraLayers.vst3 in the C:\Program Files\Common Files\VST3 folder.
Additionnally, you can check if the following registry entry exists: HKEY_LOCAL_MACHINE\SOFTWARE\Steinberg\SpectraLayers (that’s where the plugin goes to find the SpectraLayers engine).