the 15 pin connector is typically a pc thing (not saying it doesnt exist on mac but) when soundcards became the primary holder of the joystick port they tossed in midi since it was there. I don't think mac's have native raw midi input

you will have to buy some type of adapter

meantime check this out

http://archive.cs.uu.nl/pub/MIDI/DOC/Mac-midi-intro

as far as plugging your midi to pc soundcard cable (i have one too) into the mac video port, no it wont work, and its possible that it could damage your video hardware, if your midi device is pumping out 5 volts on a sensitive line