The slides for the next lecture are online: 03_xmlschema_css_xpath.pdf. We also recommend you to have a look at the tutorials about XML Schema, Namespaces, CSS, and XPATH at w3schools.com.
If you want to get practice with XPath expressions then you might want to test your XPath expressions in Java xsd-coding.zip or visit the online XPath evaluator. Here some XML you can play with: book.xml