On your Newton device:

Install the Newton Internet Enabler (NIE) to enable ethernet protocols.

Install a driver for your ethernet PC card, if necesary. See the WikiWikiNewt for further information.

Install a package that adds an ethernet connection option to the Newton Dock app. For example, EthernetDock.pkg replaces the Dock app’s AppleTalk option, or Dock_TCP-1.2-en.pkg adds a TCP/IP connection option.

Plug the ethernet cable from the PC card in your Newton device into a router or other device that provides access to your local network.
You can not connect your Newton device directly to your computer using ethernet.
On your Macintosh computer:

Launch System Preferences.
You need to allow TCP/IP access to Newton Connection through your firewall.

Click the Security icon and then the Firewall tab within the panel that appears. Allow incoming connections to the NCX application.

Close the System Preferences window.