Oops… I discovered to my dismay today that the new iTunes 10 will not play any of my midi files (.mid), which previous versions of iTunes did. I am not happy… having upgraded to 10 without a concern I would lose capabilities.
BE forewarned.
Apparently, there may be a fix and the problem has been going on longer than I realized:
I can vouch for the effectiveness of running iTunes in 32bit mode.
To use iTunes in 32 bit mode, find iTunes in your applications folder, right click to Get Info, and then select “Open in 32bit mode.” I can again play midi files…
The Eclectic Rant 
Bravo! I didn’t have a 32 bit mode option for iTunes 11 but right clicking on the icon, select properties, compatibility, run in compatibility mode for Windows XP allowed me to add midi files!