See the second post in the thread it will tell you how to add all the plugins. For Mac just search for Plugins in Help and then select Using plugins from the page and follow the instructions, Topaz plugins need to be installed manually:
Using the info you sent I managed to get the plug-ins opening from Affinity Photo. Unfortunately there’s a problem with both Denoise and Sharpen. When I Apply the modification it closes the Topaz window but actually does nothing. Then it reopens the Topaz window. Terminal reports the following message.
Both Denoise and Sharpen work fine with Capture One on Catalina.
objc[629]: Class CVWindow is implemented in both /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.2.dylib (0x10996ee88) and /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.4.dylib (0x1051cc1a8). One of the two will be used. Which one is undefined.
objc[629]: Class CVView is implemented in both /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.2.dylib (0x10996eeb0) and /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.4.dylib (0x1051cc1d0). One of the two will be used. Which one is undefined.
objc[629]: Class CVSlider is implemented in both /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.2.dylib (0x10996eed8) and /Applications/Topaz DeNoise AI/Topaz DeNoise AI.app/Contents/Frameworks/libopencv_highgui.3.4.dylib (0x1051cc1f8). One of the two will be used. Which one is undefined.
Just to be sure that you used the proper paths in Affinity Photo I post a screen shot for you!
Example:
/Applications/Topaz Sharpen AI/Topaz Sharpen AI.app/Contents/
You can change the path in the “Plugin Search Folder” by clicking “add” the panel opens and you have to do CMD + Shift + G now you can fill in the path etc.
Take a good look to my example screen shot! If all is similar it is a Catalina issue.
It’s not wise to just upgrade a OS on the Mac without checking if there are problems with the software you use!
I’m only going to upgrade when I’m a 100% sure it will all work and at the moment that’s not the case with Catalina.
Affinity & Topaz plugins work fine, even in 2018 … 2019 and 2020. See the post by @AllMediaLab above. And also Make sure that you haven’t dragged a copy of the plugins into the default plugin folder for AP.