You will need to run it as a standalone, with PS closed, for the first time and the plugins will be installed.
You can use Mask AI as an external Editor from any application but no reusable mask will be returned unless the calling application recognises the alpha channel. Most RAW editors have their own non-destructive making applications. But you will be able to return composite images with replaced background.
Well I tried that and it still wouldn’t work so I kept messing with it and I figured it out. There wasn’t a (.plugin) file in the photoshop plugin folder. So I right clicked on the app and showed the contents and in the resources folder was the plugin file. I copied it to the PS CS6 plugin folder and it now works fine. And I can also access it from Lightroom and even ON1 raw shows it too.
I had the same problem with DeNoise AI V2 (doesn’t show up in PS filters menu). I tried running as a standalone first without PS running and it still doesn’t show up in the filters menu. However manually transferring a copy of the .plugin file does work as it did before. Perhaps you should look into this a bit further.