This tutorial illustrates making a foot pedal for a Macintosh computer using an Arduino and a 2 switch guitar amp pedal.
You'll have to be a little familiar with soldering and writing simple code.
If you need a guitar amp pedal switch, you can buy one or make one with this other instructable or with this one.
Step 1: Supplies.
2) Arduino (I'm using a Diecimila)
3) Breadboard (or prototype board if you like)
4) Three Conductor 1/4" Stereo Phone Jack (got one at RadioShack)
5) 2 1k ohm resisters
6) Wire (you know whatever you have lying around)
7) Soldering Iron (and Solder will help too)