Hello Robinson,
Thank you for using Essential Diagram.
Regarding your query, you need to add the handler to the web.config file in order to render the diagram control on the web page. Please add the following code snippet into the application's Web.config file.
Please refer My Documents\Syncfusion\EssentialStudio\5.1.0.51\Web\diagram.web\Samples\2.0\web.config file to know how to add this handler tag and let me know if you have any more questions regarding this.
Thanks,
Meera.