connect to 
 macOS X

The key in connecting the messagepad with macOS X is a small program called Newton Connection (NCX).

This program allows to sync devices with NewtonOS 1 and 2 even on a modern operating system like macOS Catalina.

hardware

First you need the serial sync cable of the Newton. Since a modern Mac no longer has a serial port, you also need some kind of adapter.

Since macOS X comes with support for FTDI chip adapters, you can easily look for a cheap Serial2USB adapter with this chipset. They don't cost more than 10-20 €.

2000 / 2100

The Newton 2000 / 2100 needs the Interconnect Dongle in addition to the Sync cable. It is really hard to get and a good, but not cheap, alternative is the USB Connectivity Board. This card fits directly into the expansion slot of the Newton and so the Newton can be connected directly via micro-USB.