The other answers are somewhat on the mark, but won't work. Note that @Mike Petty's comment about matching the case in your e4x statements to the case used in the XML is one part of the problem.
However, the other issue is that e4x statements return XMLList
objects. They don't return an XMLListCollection
. So this expression evaluates to null:
_layoutTemplatesXML.PaperSizeTemplates as XMLListCollection
Instead, set the data provider for the drop down list like this:
<s:DropDownList dataProvider="{new XMLListCollection(_layoutTemplatesXML.PaperSizeTemplates.@PaperSize)}"/>
This populates the drop down list with 3 XML objects who's values are Strings for each size.