Enter a search term. In this case I am looking for files related to World War II. You could also do a search for something like "Maine Seafood." You have to enter a search term and do the search before you will see the screen pictured below.
Alternatively, you can search by file type by entering filetype: KMZ after your search term.
To clarify, you will have to open the file in Google Earth to see its contents. This method is just for finding files to use in Google Earth.
Finding files in the Google Earth gallery:
Google Earth has a lot of great layers of information built in. Simply open the "layers" drop-down menu in Google Earth and select a layer to view it (remember, the more layers you select the slower Google Earth will run). You can also open the "Earth Gallery" in Google Earth to find and add more layers to your view of Google Earth.