You must have a google Maps API Key for Maps to work.
Without a key it will be very hard to remove Google Maps errors that will warn you against using the non-registered API. You may have up to ten addresses or location pins on a single map without a registered key. The demo for OnList isusing a non-registered API and only is showing the first ten listings on the front page map.
Once you have a key, simply paste it into the OnList admin Options setting tab where you see the text field for Google Maps API Key.