this might be a bit late now, but if you are triggering keynote and power point files you might find this interesting;
http://www.keywebx.com/keyshowx.html
you can then write an apple script to trigger next, prev etc.
I can help with this if you dont know applescript -it's reasonably straight forward.
Richard has now added an interface for midi command outputs but you can also type hex directly into the custom serial and output midi that way.
Paul