Having created the ATL object, you then need to implement the IDE plug-in interface of XMLSpy. The IDL file of the project contains a new entry for a coclass. Instead of using the predefined custom interface, you can insert the definition of the IXMLSpyPlugIn interface.
The IDL must also contain the definition of the SPYUpdateAction enumeration. Also, replace the generated interface name, (created by the wizard) with "IXMLSpyPlugIn" in the coclass declaration. The IDL should then look something like this: