Magnific is installed.

One manual step remains — Affinity only loads plugins from folders you add yourself:

1. In Affinity: Settings (⌘,) › Photoshop Plugins
2. Under Plugin Search Folders click Add and pick:
   ~/Library/Application Support/Magnific/Affinity Plugin
   (in the file dialog press ⌘⇧G and paste that path)
3. Tick Allow unknown plugins to be used
4. Restart Affinity — Magnific appears under Filters › Plugins

To uninstall, delete that folder. The filter needs an RGB/8-bit document with a pixel layer to be selectable.