Just a simple note for anyone trying to change the style properties of an element in Adobe Flex: use the .setStyle method on the element you want to change:
progress.setStyle("paddingRight", (event.bytesLoaded / event.bytesTotal) * 300);
And on the same track: If you want to place the items in a box close together, simply use the horizontalGap and verticalGap attributes on the mx:Box element to remove any spacing between the UI elements.