Here's how I measured the circumference: While I straddled the bike in a flat parking lot, my wife marked the starting point with a chalk line on the tire and the pavement. I then rolled the bike straight ahead for two full revolutions, and my wife marked the pavement where the mark on the tire showed the second full revolution. Then we measured the distance between the two marks, divided by two, and had the circumference.
The tricky part was programming the unit. As I recall there is a delay between pressing the button (per the directions) and when the unit would register the input. It took some trial and error and frustration to get the hang of it. Once it's set up it works great. I just wish it had a light so I could know my MPH at night.
Some helpful info here:
http://houseofthud.com/bikecomputer.html
