I am EXTREMELY frustrated! I have spend my ENTIRE Sunday debugging this, but I can't get it to work. I have all the connections correct, I'm pretty sure. I am using a split-end cable which goes right into the female jack on my microKorg synth. I am using a 4N28 opto-isolator. The 220ohm resistor connects to pin 2 on the other end of the cable (the circular, male metal connector end end). To help myself visualize, when looking at the end of the male MIDI jack, I number the pins:
2. 3. 4.
pin 4 on this cable connects to pin 2 of the opto. I am NOT using pin 3 (which is GND for the MIDI connector). I shouldn't be, correct? Anyways, with or without, it hasn't worked.
I read the details on using a different value resistor on pin 6 of the opto. For some reason, I SOMETIMES will get a '1' returned with Serial.available(), IF i mess with a potentiometer between 100Kohms and 200Kohms (usually ~170 does it), but it isn't reliable at all. I've got this all set up on a breadboard for now.
It is my understanding that this is supposed to connect to the jack on the MIDI device labeled "MIDI OUT", correct? I am also getting a little confused with perspectives here lol (flip-flopping cable/jack pins 4 & 5 to match male/female jacks/sockets). Have I possibly blown out the optoisolator? I don't see what I'm doing wrong! 
FYI: I have been referring to the original schematic on page 1 of this topic.