To answer a question that seems to have disappeared, you can route MIDI to the Quicktime synth on OSX easily enough. Be aware I'm still on 10.6 so some things may have changed on newer versions.
  • Open Audio MIDI Setup (from /Applications/Utilities).
  • Now look at the MIDI Studio window. If it isn't open, choose Show MIDI Window from the Windows menu.
  • Double-click the IAC driver to show its properties. Make sure there's at least one IAC bus in the Ports list in the lower left. If there isn't, click the + below the list to add one.
  • Run mess -listmidi and make sure you can see the IAC bus(ses). If you can't, check Audio MIDI setup again, if you can, close Audio MIDI Setup.
  • Get MidiPipe and extract it somewhere.
  • Launch MidiPipe. You'll get a window with a list of "Tools" on the left, and an "unnamed pipe" with no entries on the right.
  • Drag a "Midi In" from the Tools to the pipe and select it to show its properties at the bottom of the window. Set its MIDI Input to an IAC bus, and don't select "hijack".
  • Drag a "DLS Synth" from the Tools to the pipe, below the "Midi In" you added in the previous step. The default properties will use QuickTime synth, channel 10 mapped to standard kit and the remaining channels mapped to piano 1, and a GS-compatible program map. You shouldn't need to change this.
  • Make MESS send MIDI to the IAC bus you selected for the "Midi In" a couple of steps ago.

If on the other hand you need to send MIDI into MESS, you'll need to go a different way. You could use the "SMF Player" and "Midi Out" tools in MidiPipe to play a MIDI file into an IAC bus, or you could get a soft keyboard program.