Getting Nik to work with Affinity is complicated because Affinity recognises only Photoshop plug-ins, but Photoshop plugins only get installed if Photoshop is detected on your computer! I really wish Serif would address this...
Luckily, Nik can be tricked. 1) If you have already installed Nik, uninstall it. 2) Go to you your Applications folder, and create a new folder called 'Photoshop Plugins'. 3) Run the Nik installer 4) When you get to the 'Compatible Host Applications' screen, where you see Aperture or Lightroom or whatever listed as detected applications, you will see a '+' at the bottom of the screen. Click this, and it lists everything in your Applications folder, included the 'Photoshop Plugins' you just made. This should not be greyed out (because it has 'Photoshop' in the name...) so you can click it and add it as a compatible application. 5) Continue to install Nik. 6) Go to Affinity > Preferences > Photoshop Plugins 7) Add you Photoshop Plugins as a search folder 8) Restart Affinity As the Nik Photoshop plugins should have been installed in your Photoshop Plugins folder, they now be available in the Filters/Plugins menu. Most of them work, but you get some error messages warning that it modifies the active layer instead of creating a new one.