Editor In Chief Paul White replies: While you can do some neat 'drag to connect' things in OS X's Audio MIDI Setup window (which is found in the Utilities folder in Applications), adding MIDI instruments to Logic is still best done in the sequencer's Environment window. You can set up and test MIDI ports in Audio MIDI Setup (AMS) just by scanning your connected interfaces, and all available MIDI ports show up in Logic. But there are lots of things you can't do in AMS (or at least I can't find them!), such as setting up Multi Instrument object Parameters, MIDI Bank Change message protocols and patch names, so I'll explain how to set up a MIDI instrument in the Environment window.
First, if you're not on the MIDI Instruments Layer already, select it from the menu just beneath the Toolbox. If you can't see the Toolbox, use the View menu to turn the Parameters view back on. Once you're in this page, go to the New menu and select New Multi Instrument. This will create a new Multi Instrument object with 16 squares corresponding to the 16 possible MIDI channels, though it doesn't matter if your instrument has fewer parts than this, or even if it isn't multitimbral at all. Initially the squares will have diagonal lines through them, so you need to click on the ones you wish to use with your instrument to make them active. For example, if you have an eight-part multitimbral synth that is set to operate on MIDI channels one to eight, then click on boxes one to eight to get rid of the diagonal lines. Now you can click on the Multi Instrument name at the bottom of the icon and use the Text tool from the Toolbox to open up a name dialogue box. Give your object a name relating to the physical synth you have plugged in.
With the Multi Instrument object still selected, go to the Parameters box at the left of the window and select the physical MIDI port to which your synth is connected. It's also worth selecting each of the squares you've unticked and then ticking Program, Volume, and Pan in the parameter box so that your connected instrument can receive these MIDI messages from Logic. Why these are off by default I'll never know! If you select your Multi Instrument by clicking at the top of the object, the parameter box on the left should show All in the MIDI Channel information line. If you select by clicking at the bottom of the object, the MIDI channel of whichever numbered square is currently selected will show up in this line.
You can also change the graphical icon assigned to your Instrument by clicking and holding the mouse pointer over the existing icon in the Parameter box. There are plenty to choose from, and selecting a suitable icon for each device can help you keep track of what's going on in the Arrange window. The icon box needs to be ticked if it isn't already. A message will then ask if you'd like this icon to apply to all the MIDI channels of your instrument. Normally this is fine, but it's up to you.
Finally, if you double-click on one of the numbered squares you've switched on, a patch window will open up filled with General MIDI patch names. In this window you can choose the MIDI Bank Change message required by your instrument from the Bank Message menu, and you can also type in new patch names to match your actual synth. This is pretty tedious, but a quick Google search will almost certainly locate a ready-made Environment object for your particular synth, in which case just cut and paste its Multi Instrument object into your Environment before you complete all the steps that come after creating your Instrument object. Needless to say, you should do all this in your default or template song so you only have to do it once. To add the new Multi Instrument object to an older song that you're resuming work on, simply copy and paste it from your default song and then make sure it is assigned to the correct MIDI port in the Parameters box.