You need to set the model to display.
The model can be placed on any HTTP server. The idea here is that the widget makes a GET request to get the model, so the url for ModelUrl needs to be something that can receive GET requests and answer directly with the file. Putting it on Google drive may not work (haven't tested it though) since when you click on a google drive link you usually land at a google page and only after that you click download. Personally I recommend putting it on the same server that hosts Thingworx. Also, you can host the models directly in Thingworx, by uploading them into a FileRepository, or as Media Entities.