How can I access to a XML document object hierarchy?

The most common way to work with XML documents in AppleScript is using the scripting additions XML Tools and XSLT Tools, from Late Night Software, in conjunction with its XML-RPC Lib.

They are able to translate XML documents into records and back, and use XSLT stylesheets for transformations.

AppleScript also supports XML-RPC calls and SOAP requests. More info, here.

You can also find some useful AppleScript libraries for XML parsing here.