On a page in your modeler, Right click, press Add Widget, and search for the HTMLSnippet widget
Preferably you use the App Store integration in the Desktop Modeler, like Paul suggested.
When you download the mpk file you have make sure you move it to the widgets folder located in your Project root.
To sync all the widgets in your project folder with the Desktop Modeler you can use `F4` (Synchronize Project Directory)