The following has worked for me in the past from the command line:
Kill any running WSJTX
1. Open Terminal
2. Type: sudo rm -rf /Library/LaunchDaemons/com.wsjtx.sysctl.plist
3. Authenticate with your Admin password when prompted. Verify that /Volumes/WSJT-X/com.wsjtx.sysctl.plist is no longer in that directory
5. RE-Mount the wsjtx.dmg when your machine is back up again
6. Copy over the latest version to /Applications (just to be sure it is the latest in /Applications)
7. Run sudo cp /Volumes/WSJT-X/com.wsjtx.sysctl.plist /Library/LaunchDaemons (again)
8. Test by launching WSJX.
I found that the Daemon cannot be overwritten while it is in use (loaded). Hence the delete/reboot/try again.