If Powerpoint is running on Windows, the simplest solution would be to use Gobetwino to send keystrokes to your applications based on command strings sent from the Arduino. Gobetwino isn't available for non-Windows platforms (as far as I know) but it would be easy enough to write your own application to generate keystrokes.
If you aren't committed to making your own hardware, you could probably get a wireless mouse for less than the cost of an Arduino.