There are two general methods for embedding an icon into a Button control. The first method involves creating a variable in the Flex application, and using the [Bindable] and [Embed] meta data. You specify the path to the icon in the source parameter of the Embed meta data and then to use the icon in a Button, you use data binding, which in this case would look like icon="{Icon}". The second method, embeds the icon inline in the Button tag itself, by using @Embed() and specifying the path to the image file as a parameter.
See the full code after the jump.