This post explains how to run the XML validator to use a specific schema in XML ValidatorBuddy to check XML.
If you use the “Validate against a specific Schema…” functionality of XML ValidatorBuddy the editor looks for a schemaLocation or noNamespaceSchemaLocation attribute in the XML instance file. The validator also checks the selected schema if any targetNamespace is defined. If a schemaLocation or noNamespaceSchemaLocation attribute is present in the XML instance file, XML ValidatorBuddy overwrites the values (of course without modifying the file) of those attributes to assign the selected schema for the XML validator. If no schemaLocation attribute is present in the XML the target namespace of the schema is taken.