If you have an earlier version of PowerPoint, there are some other
applications that may be able to assist you.
You can use the free iRedLite application to do that:
<http://www.filewell.com/iRedLite/>
PowerPoint 2008 allows you to use the Apple Remote Control for slide show. You can use the Play or Skip Forward to go forward and Skip Backward to go back one slide or animation iteration.
The + and - buttons can be used for system volume control within a slide show.