1. Set resultFormat="e4x" on your HTTPService
2. E4X expressions don't reference the topmost tag of the XML
3. Use for each, not for
4. Use for each (child in parent.*), not for each (child in parent)
5. E4X intentionally blurs the distinction between XML and XMLList

For more details see:

http://www.morearty.com/blog/2007/03/13/common-e4x-pitfalls/