Per the recent request for Macintosh-specific MIDI applications,
I use the following two applications with 100% success and ease:

#1 "Aria Maestosa", a free piano roll editor for Mac OS X 10.4,
Windows XP+ and Linus (Alsa), available at
http://ariamaestosa.sourceforge.net/ 

#2 "MidiKit", for all kinds of file conversions, available for
about $15 at http://mixagesoftware.com/ 

Hope this helps!

Gordie Davidson
http://www.youtube.com/user/grindergordie 
gordiedavidson@gmail.com.geentroep [delete ".geentroep" to reply]