The "midi in" and the "midi out" field of the IAC Buses can not be edited in the Audio-Midi setup, it is greyed out.
The help says you can determine the number of midi in and midi out channels here.
How do I activate the field, so that the number of midi channels can be edited?
There is a "+" and a "-" button below the ports window.
Add to your heart's content!
This adds another IAC Bus here, but the midi channel can not be changed.
Did you try this on your machine? Did it work, can you change the midi channel?
In Audio Midi Preferences, is the IAC icon red? (meaning turned on) ?
Retrieving data ...