the topic is confusing, but you want to use arduino as controller for other midi equipment, right? in this case i would speak of arduino as midi output device (because arduino puts out midi). anyway! in case you want arduino speaking midi, you can use these circuits
they are taken from here:
http://www.tigoe.net/pcomp/midi.shtml
please note, that the midi input circuit (that means that arduino receives midi) is discussed here:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1152354897

my personal experience for arduino as midi controller so far is that timing-critical applications (as e.g. a step sequencer in my case) are not working well, as long as you don't use timer interrupts. i am still working to get that running properly
easier tasks should be less critical, e.g. changing a midi continous controller by the readings of one or several flex-sensors.

Hello, I recently was trying to get the MIDI IN ciruit to work but my optocouple output only changed .1 volts when I applied voltage to the midi jack? Can someone help?

Thanks

Nigel