You will need to install the latest version of Experience Builder - Developer Edition to your computer prior to installing the custom widget. For instructions on how to install please click here https://developers.arcgis.com/experience-builder/guide/install-guide/
Copy and paste the ‘multi-oic-viewer’ folder in the following location: ArcGISExperienceBuilder\client\your-extensions\widgets
When you are working on your experience, you will see the Multi OIC Viewer Widget at the bottom of the widget panel, under the custom option.