Apple really obfuscates drivers in Mojave in about the worst way possible. I'm unsure if this method works for Catalina as I've yet to upgrade, but my guess is probably not.
I personally only have the NS7s, but this should work for the NS6, N7, NS7FX as well as they all share the same drivers.
- Install the drivers
- You should see a System Extension Blocked message
- Go to system preferences first thing, and then click Security and Privacy
- Click the Allow button. This will bring up a list of extensions, check the Serato drivers, and look for any drivers that end with GmbH (low latency drivers) and check those as well and click ok.
- The Numark drivers come with a utility that can detect if the NS7s are connected, they also should show up in your control panel as well.
Congrats, that's it, now you can use your Numark N series in Mojave. Serato DJ Pro is free for as well.