I do not remember if there is a parallel port on a MAC computer, if so you can program it, using just one bit, probably basic is the most common language used on the OS.
then using some IC to isolate the port from the high current demand is necessary. In this site there are some hints, look for relay in the searh engine.
Why don't you buy a computer board from Zilog the are sold for about 49 dlls. including the development software, here you can have the flexibility to create your own projects, ADC, DC motor control, etc.
Complex systems like Mac's are pretty good to learn, however you need to know basis from the architecture, addresses locations, etc.
I now how to program in different language's and a board how you descibed is to simple also microchip.com is the website of PIC microcontroller which I use alot.
But I do not wish to use a PIC microcontroller for this project.
Also what do you mean about 49dlls what does that mean is it dollars?
Not to be picky (I'm assuming this is an older Mac if it's running System 6) but db25 port on the back is SCSI, not parallel. There is a serial port, and that could be wired to a latch or whatever....