Thanks for you answer.
The reference is to the class element and not the package.
And the Utiliies package is used in the EAExample model in the package "Model Based Add-Ins"
I am building a model addin using the same structure.
After testing, it seems that the example is also not functional.
The AddIn "MyDemoAddin" has the reference to 2 class elements, but the operations calling them (TestOtherObject, TestXML) is not implemented.
After adding them:

they throw the same error:
Invocation error in: addin.EA_MenuClick
------------------------------------------------
344: }
345: MyDemoAddin.prototype.TestXML = function() {
346: var doc = new JSxml();
347: doc.SetCurrentNode(doc.CreateRootElement("properties")); [!!!! JSxml is not defined]
348: var group = doc.AppendChild("group","");
349: doc.SetAttribute(group, "name", "theGroup1");
350: this.MultiLineOutput(doc.GetXML());
351: }
352: MyDemoAddin.prototype.UpdateSelectionSummary = function(Repository) {