It's the same as the Admin and SMDR ports on the Magix processor, so use the same 355A adapter you're using there. Fire up Hyperterminal and set the parameters to:
§ Bit per second: 38400
§ Data Bits 8
§ Parity: None
§ Stop Bits 1
§ Flow control: None
Hit return, log in as "sysadmin" and use the same numeric password as the System Administrator's mailbox.
You can also dial into the Automated Attendant and dial "**3" to transfer to the internal modem.