I think you should take a look here: https://docs.mendix.com/howto40/custom-widgets
You can open the App store in the Mendix Desktop Modeler.
If you then navigate to / search for the specific widget, you can click Download and it will automatically be installed in your project.
It is then available in the Add-ons menu in the Page Editor.
1. copy .mpk file under yourProject/widgets folder.
2. Goto Modeler project and press F4 or Project Menu-> Synchronize Project Directory
3. Goto toolbox, scroll down at the end 'Add-on widgets' to find new widget.