In case you didn't figure it out and for future visitors: you'll need to use the IP address listed in the Plugins or Jails UI for SABnzbd, which for me was 172.16.0.10/30 (exclude the /30 when you're entering it in Sonarr). Also, you'll need to use the port specified in SABnzbd (which defaults to 8080), not the external port set for the jail.