Was the integration with Balsamiq retired at some point? I still find articles saying that you can easily add mockups on ux.sx using a built-in icon, but I don't see the controls anywhere in the editor.
It wasn't officially retired, in fact it's still kind of available. But it's built using Flash, and considering no modern browsers support Flash any more it's not a feature that can really be used.
Perhaps if you have an older browser you can test it out to see if it still works? It's the Wireframe icon next to the Image icon on the main site.