" from the reference page"
Which one? I couldn't find it.
I only found the http://processing.org/reference/XML.html page.

Sorry, yes, I should've included the link. It's here:

Mmm, I made an answer yesterday, but it was lost, somehow.
I couldn't read your code because it is on one line (Zoho behaves curiously sometime with HTML formatting, better paste plain text).
So I missed that parseXML() was a top-level function, like loadStrings() for example.
Indeed, it is in the reference, but not in the code at the time of 2.0b7: https://github.com/processing/processing/blob/processing-0215/core/src/processing/core/PApplet.java
Looks like the reference is so up-to-date that it references the current state of the code, not the state of the released version...
Anyway, you can still parse the XML with some changes to the sketch:

1. String data = "<mammals><animal>Goat</animal></mammals>";
2.  
3. void setup() {
4.   XML xml = null;
5.   try {
6.     xml = XML.parse(data);
7.   } catch (Exception e) {
8.     println("XML could not be parsed.");
9.   }
10.   XML firstChild = xml.getChild("animal");
11.   println(firstChild.getContent());
12. }