To set up a Macintosh so it will play Midi files, you will need Netscape 2.0 or later, a midi plug-in and Quicktime and Quicktime Musical Instruments 2.0 or later.
Put the plugin in the navigator's "plugin folder" then follow Arnold's instructions for enabling it and properly setting mime type. If using Quicktime 2.5, use the "QT settings" control panel to select QT midi or an external midi device.
My own setup is:Quicktime and other system software updates can be found at Apple Support and Information.
Arnold's Midi Player Home Page is the source of an excellent midi file player and includes good links to midi information. See also "Interesting Links" on my home page.
For Windows machines: Try MidiGate at http://www.prs.net/midigate.html, a midi plugin for Windows. This site gives instructions on how to set up the computer using a sound card. Quicktime for Windows is available at Apple Support and Information.