If you add a doctype to your xml and use a XML editor (i.e. JEdit with XML plugins) you can easily see whether the XML is well formed or not.
Also you might have a look into the JBOSS/dosc/schema directory where the schemas are located.
If you look to 'jboss-web_5_1.xsd' you see a simple example.
The class-loading element allows one to override the default class loading behavior of the web container.
Examples:
<class-loading java2ClassLoadingCompliance='false'/>
<class-loading java2ClassLoadingCompliance='false'>
<loader-repository loaderRepositoryClass='dot.com.LoaderRepository'>
...
</loader-repository>
</class-loading>
For more details about classloading see the JBOss wiki
My tip: surround your loader-repository with <class-loading> as you can see in the Exception.