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)
Step 2: Solder the Jack
The Sleeve (attached to the black wire in my photo) is the common line running to each of the switches on the pedal. If we give it (the black wire) a voltage, switch #1 will turn on/off the Tip wire (green in my photo). Likewise, switch #2 turns the Ring on and off (yellow in my photo).
If you get confused (I kinda did) just solder a different color wire onto each lead on the jack. You can use a volt meter to test for continuity (or wire it up to the breadboard with an LED on it) to figure it out. I kinda looked at each lead and figured out what part of the plug it actually makes contact with.