I need to create a GUI extension in which after select one or more elements and click a ribbon button a popup window should appear so show the user some options and after seletec them and click ok a workflow has to start. Right now I'm focused on trying to open the popup window and pass to it the selected items. I created in the js file the following code:

Extensions.enviarTraducir.prototype._execute = function enviarTraducir$_execute(selection) {
var tcmUris = "";
for (var i = 0; i < selection.getCount() - 1 ; i++) {
    tcmUris += selection.getItem(i) + ",";
}
tcmUris += selection.getItem(selection.getCount());
var host = window.location.protocol + "//" + window.location.host;
var url = host + '/WebUI/Editors/Traducciones/client/html/popup.htm?Uris=' + tcmUris;
var popup = $popup.create(url, "toolbar=no,width=400px,height=200px,resizable=false,scrollbars=false", null);
popup.open();
};

but it still doesn't work. A part of that if it works, I don't know how pass that information to a core service code that makes the workflow starts and do his stuff. Does anyone has a clue, links, tutorial or something it could help me? Thanks in adnvance