Hi Sam
There are a number of ways one can do this, none of them perfect.
First off, canibalise a USB mouse, and connect the mouse buttons to a DMX relay board or box, and then by just setting a channel to full for half a second you can get the next slide up.
I think that Screen Monkey (free, from
here can display PPT presentations, and if it can do this to your satisfaction, then the Professional Edition (£120) can be triggered from DMX, and so should do what you want.
If a Ma can send strings out of the RS232 port, then another possibility is
TriggerPoint.
Let us know what you decide to do, or if you find another solution.