jOrgan is an open-source, free, virtual organ, which can be run on several platforms. Most of the people using it seem to be running Windows, and some of the Mac users have had trouble getting it to run, so I hope that these notes might be helpful. I have not done extensive testing with a lot of different configurations, so you may need to be adventurous in your own setup, but good luck!
I have used it successfully under 10.7 (Lion) and 10.9 (Mavericks) on an iMac and a recent MacBookPro. From the looks of it, the methods described here ought to work on Macs running recent versions of OS X. I have worked only with dispositions (the virtual organs made to run in jOrgan are called "dispositions") that are made for FluidSynth (see "basics", below), but I'm guessing if you run into a disposition that is intended for some other synth you may be able to figure it out in a similar way to what I did.
jOrgan requires its sounds to be played through a software synth of some sort. The dispositions I looked at were for FluidSynth, also an open source project. Fortunately, FluidSynth is included built-in to the OS X binary distribution of jOrgan, which was version 3.20 at the time of this writing [Fluidsynth can be installed and used as a stand-alone system, but that is an entirely different matter from what I have discussed here, and not necessary if all you are doing is trying to play organs through jOrgan].
To make the dispositions that I downloaded work on OS X, a few minor changes were necessary, as described below. You might want to try it out on a copy of the disposition called "fluidsynth-sample.disposition", which is supplied as part of the installation. If you can get this one to work, then you should be able to download other fluidsynth-based dispositions and have them work on your Mac.
IMPORTANT: These modifications worked for me but I cannot guarantee that they will work on your system. Use at your own risk, as I will not be responsible if they don't work for you, or cause some other pain. If they don't work, maybe they'll suggest something to you that will work for you, though.
I STRONGLY advise you to always work on copies of the original files so that in case something goes wrong, you can start over with a fresh copy.
OK - finally here's the fun part, and the guts of what makes the whole thing work!
There is one other possible source of trouble that I have seen in some of the dispositions that I have downloaded, so if the steps above do not result in an instrument that plays from the onscreen keyboard, you may need to look further. Try going back into construct mode, and checking in the consoles folder, and look for an item with "connector" as part of its name, usually right at the top of the list. Click it and see what is next to "output" in the properties list. Sometimes it has something else there that may not exist on your system, but for Mac OS X it seems to work fine when left blank:
So at this point, your jOrgan disposition should be able to play through your system. However, you will still need to be able to route the MIDI data through to it. The way to do this will depend much on what kind of system you are using. At the moment, I do not have a MIDI keyboard or interface available to test with, although I have some ideas about how this might be done. I'll try to update this document later, when I am back in my regular location with the requisite equipment at hand. However, I have been able to make jOrgan work with the musical notation/playback program, Finale®. If you are interested in that approach, then read on!
Finale® is a music notation and playback program, and jOrgan can easily be used as the playback instrument in Finale®, with the obvious set of advantages that jOrgan provides over the few organ sounds available in the stock Finale® installation. I have tested this method with Finale® 2012 under OS X 10.7 (Lion) and 10.9 (Mavericks), and would expect the procedure to be similar and to work as well with the various other combinations of recent versions of the two products.
Assuming that you have a working jOrgan disposition, as described above, here's the procedure to play Finale® scores through jOrgan:
update history:v-1.1 - Removed the Mavericks hack for the Gatekeeper message, and replaced it with the better and more general Gatekeeper procedure.
If you are running jOrgan under a version of OS X with Gatekeeper (10.8 and above), you might get a message saying that "jOrgan is damaged and can't be opened. You should move it to the trash".
The problem is that the Java stub used in the jOrgan binary distribution is unsigned, so Gatekeeper thinks it is from an unknown developer and wants to protect you from a potential danger. Since we trust jOrgan, we can temporarily disable the protection, teach Gatekeeper that this application is safe, then turn the protection back on.
NOTE: The protection provided by Gatekeeper is there for a good reason. Don't defeat it unless you know the implications of what you are doing, and are sure you can trust what you are trying to run, or beware the consequences, which can be dire!
That being said, here's how to fix the problem for jOrgan: