Step 4: Programmin'
So, you could use MAX/MSP/Jitter or Pure Data to process the signals from the Arduino, and that might actually give you more flexibility (I'll try to post some info on this soon), but instead there's a really great little application out there called Serial-MIDI Converter. It does just what you might think - converts signals sent over a serial connection to MIDI. It works with any MIDI-compatible program such as Garageband or Ableton Live. Click on the link and download it, all the info on how to use it is right there on the download page.
Like I said, if you've never used or programmed an Arduino before, you really should check out the Getting Started section on the official Arduino site, as well as their tutorials.
The code is commented, so I won't explain it in detail, I'll just post the sketch here at the bottom of the page. Basically, you just set a lower and upper threshold for each sensor. If the upper threshold is exceeded, a MIDI message is triggered, and once the force sinks below the lower threshold, another hit can be triggered. It's called hysteresis, and it makes sure that you don't set off multiple messages per hit.
You may have to fiddle around with the thresholds, depending on your personal foot-tapping style and weight distribution.