You can convert the KMZ file to a KML file through Google Earth (load the data, then Save Places As...) or unzip the file through whatever works on your computer and operating system.

But we don't want a KML file, we want a geojson file so it's easier to work with in javascript. Again, there are lots of ways to achieve this. The least technical way is to use an online conversion tool.

This solution also uses a Leaflet plugin called leaflet-ajax to load our JSON file directly without having to assign it to a variable first. The code here borrows very heavily from the provided example.