You need to login with Open ID to wiki (click "Log in with OpenID" on the top-right).
Before you need to have Open ID account. if you dont have it - register it for example: here
(btw: according to existing xml functions it will be better to name your function xmlGetNodes)
after login go to:
https://wiki.multitheftauto.com/wiki/XmlGetNodes
and press "edit" link.
if you dont know anything about wiki syntax, you can check another function wiki source (like this one - result)
fill your function page, and save it. you are done.