Save yourself the time and just use FlashDevelop. There's no need for the MXML files. You just use .as files.
http://www.flashdevelop.org/communit...pic.php?t=1592
That's the newest build. It runs just fine. If you choose to use flashDevelop all you have to do is install it then goto tool->installed plugins then click on AS3 context then click settings then in the Flex 2 SDL location set the folder where the Flex 2 SDK is. Then when making a project (by clicking Project->New Project and selecting Empty AS3 project) you must set the debug drop down to release and go into: project->properties then at the bottom of that window it says, "open swf in:" set that to external.
There you're done.
Another thing to note when you make a project in the project outline there's a button that says show hidden items, click that. Then for your main class that you create remember to extend Sprite and right click on it in the outline and select "Always Compile"
The IDE also does XML and other languages with highlighting I believe built in.