Basically, Apple has designated the iPhone/iPod Touch General Settings as the place to make "set once" choices that affect the behavior of an application. This is where the kilometers/miles setting can be found.
On the home screen, there is an icon labeled "Settings" that is gray and looks like a gear. Open "Settings" and scroll to the bottom of the screen. There you will see a selection for each application you have installed that has "set once" choices. MileBug will be listed, and by selecting it, you will find the following options:
1) Open To: (Trips List or Add Trip)
2) Measurement: (Kilometers or Miles)
So to use kilometers, simply select kilometers for the Measurement setting and return to MileBug.