It seems you try to load and evaluate a sitemap.xml wit seleniumIDE.
The way that works for me was to use javascript in combination with assertEval like this:
1)
open
URL_TO_YOUR_SITEMAP.XML
2)
assertEval
javascript{window.opener.content.document.getElementsByTagName('urlset').length}
1
3) // what you want
store
javascript{window.opener.content.document.getElementsByTagName('urlset')[0].children[0].children[0].textContent}
firstUrl
4) // optional: a direct assert without using a variable
assertVal
javascript{window.opener.content.document.getElementsByTagName('urlset')[0].children[0].children[0].textContent === 'http://www.someurl.com'}
true
The actual trick is to use
window.opener.content.document
to access the opened document.
If you use just "document", the XULDomument of SeleniumIDE will be accessed