Ricardo,
Take a look at this simple transformation example,
http://help.sap.com/abapdocu_70/en/ABENABAP_ST_EXAMPLE.htm
http://wiki.sdn.sap.com/wiki/display/Snippets/XML XLS transformation
Last, but not least take a look at the blog series here in SDN.. He has a five part series for XML processing in ABAP...
/people/tobias.trapp/blog/2005/05/04/xml-processing-in-abap-part-1