Solved using Topaz plugins with Affinity Photo on Mac OS 10.13.2 High Sierra.
- Install Topaz plugins as per instal instructions for Photoshop plugins.
- Open Affinity Photo
- Click the Afinity Photo menu item and select Preferences
- From the root (/) directory on the MAC select the LIBRARY as the Plugin Search Folders field
- Select root (/) directory as the Plugin Support Folders field NB: The Authorise Global option is automatically selected (greyed out).
- Check the Allow “Unknown” plugins to be used" option
- Restart Affinity Photo