If you are talking about turning an XML attribute into an argument, this is turned off by default. Depending on what protocols you are using, you might want to set the lisa.magic.string.xml.tags property to true in your local.properties file, and restart DevTest. If this does not work, try adding the Generic XML Parser to your list of request DPHs, and select the XML attribute you want to target in the editor window. In either case, you will likely want to regenerate your Virtual Service to account for the new argument that will be created from the XML attribute(s).
Regards,
Ken