Mac MIDI goes to the serial port, not ADB. There are lots of designs out there for making your own interface, or you can hunt one up on ebay for not too many dollars.

Forget ProTools - it's for audio, not MIDI, especially versions that old. There are older versions of Logic and Cubase that will run on a 5300, and you might want to check out the shareware MIDIgraphy.

Check out http://www.hitsquad.com/smm/mac/