I usually use the "add-on kit" to get the interface files for reference, etc, but it seems that the latest (enUS) package is missing some of the default "add-ons". Has it not been updated or is there some secret LRLRABABSTART combo I have to press? :P
You start your wow executable with the -console command,
then bring the console up (the tilde key ~ is the default for windows, not sure about mac)
and type 'exportInterfaceFiles code|art' (no quotes) to export either the lua/xml files or the art assets.