Flash doesnt support padding, these are all the supported styles:
http://www.adobe.com/livedocs/flash/9.0/…StyleSheet.html
But you can fake it, by setting background to false, and then create another background (an image or a textfield without text) have the background have a lower zorder then the textfield above it. make the textfield (for a padding of 10px) 20px less wide, and 20px less height. Make textfield a parent of the background, and then give the textfield an offset of 10px on x and y.
so:
<plugin name='textbakckground" url="textfield.swf" background="true" backgroundcolor="0xFFFFFF" width="400" height="400" />
<plugin name="textfield" url="textfield.swf" background="false" parent="plugin[textbackground]" x="10" y="10" width="380px" height="380px" />