KML files offer a convenient way to pick places on the globe. The Premium Google Map block lets you upload a KML file and display that place on your map. You can either upload a KML file through the block edit interface, or select one you've already uploaded to the file manager.
Google Earth makes it easy to generate KMLs. For help using Google Earth to generate a KML, please read Exporting KML.
For more in-depth informaiton on KML files, see this KML tutorial by Google.
Make your KML file accessible
Google's servers need to read the KML file you've uploaded to your website in order to return the correct map image-- otherwise you'll just see the default view (zoomed out high above a map of the Pacific Ocean). If you're using this block on a local installation of concrete5, Google won't be able to read the KML.
Why not? Home computers aren't usually set up to accept incoming connections in the same manner as a real web site. Development platforms often use temporary urls like http://localhost/ that are configured specially to work on your computer-- but they aren't accessible to other servers on the net. Similarly, broadband modems and the ports on your computer's operating system may not be set up to allow incoming requests.
To make the block work, you'll need to run it on a real, live website that is accessible to the public, over the internet.