Double Negative & others interested in using PS plugins in the Affinity Photo software - here is an extract from the discussion on their forum about how to use plugins:
"First, you have to enter the paths containing your plugin folders in the Plugins Search Folders section of the Preferences -> Photoshop Plugins dialog box.
Next you have to enter the support path for those plugins in the Plugin Support Folders section of the same dialog box. As a general rule we have been told to use the / path since the proper path may differ for individual plugins.
This should allow you to be able to call your plugins and they may (or may not) work. Plugin support is still an ongoing effort and it has been hit-or-miss. Personally I find that I can call my Topaz plugins and some of them work and some of them do not work. However there is an update coming which we have been assured by the developers will fix the plugin issues. I believe that they have been concentrating on core editor functionality and leaving some of the other functionality to the end. I am reasonably sure that they will have it fully working in the next RC release or the one after that so I expect full functionality within the next week to 10 days. Note, however, that I am only a user, not a Serif developer or forum moderator so I may be wrong about the date for full functionality."
And here is a link to the complete discussion of this topic on their forum:
https://affinity.serif.com/forum/index. ... p-plugins/Hope this helps.