Several major MultiSelect features were brought out through the years, which forced us to introduce some breaking changes in the widget implementation:
Indeed, the jsBin demo does not work with the latest versions of Kendo UI due to the changes that I've already mentioned. In general, we do not guarantee backward compatibility to the custom solutions (not officially supported) shared in the forums and the developers should maintain the code on their own.
For your convenience, I am sharing a how-to demo that demonstrates how to add new items into MultiSelect:
The demo uses the latest official release of Kendo UI. You can use it as a reference to accomplish your business requirement.
As a side note, we are working on a better solution exactly for such integration between popup widgets and the need of CRUD operations. You can find more details here:
Telerik by Progress