FidoNet worked as follows: User runs terminal software and dials his local FidoNet BBS system. Reads his messages, types new messages to other FidoNet subscribers and logs out. Late at night when long distance rates are low, FidoNet BBS PC dials via modem a neighboring FidoNet system or 2 or 4, passes on saved messages going that way, and receives and stores incoming messages and messages in transit passing thru on their way elsewhere. Someplace in the system is a FidoNet BBS with a terminal account on a Unix system attached to the internet. That system dials the Unix host, opens a telnet session on the Unix host, and sends mail intended for internet addresses, fetches mail addressed to FidoNet, and logs off. FidoNet mail is passed along to other BBSes as they dial in.

Since I doubt any FidoNet BBS systems are still running in your fair city, you'll have to skip that part. If you can hook your Mac to a serial adapter on your OS X Mac and learn to use Unix terminal commands, you can log onto your Unix Mac via telnet and send / receive mail etc. from your 128K. You'll have to use terminal software on the 128K. You can do anything you give yourself permission to do on your OS X Mac within that terminal, just like opening a terminal on the OS X Mac itself. None of this has any GUI interface - everything is text and command line based just like this Wiki. (Well this does allow smilies, and they are graphics.)