You have added the add marker method call outside the function and that causes it to execute before the initialize method which will be called when google maps script loads and thus the marker is not added because map is not initialized
Do as below....
Create separate method TestMarker and call it from initialize.

<script type=text/javascript>



    // Standard google maps function

    function initialize() {

        var myLatlng = new google.maps.LatLng(40.779502, -73.967857);

        var myOptions = {

            zoom: 12,

            center: myLatlng,

            mapTypeId: google.maps.MapTypeId.ROADMAP

        }

        map = new google.maps.Map(document.getElementById(map_canvas), myOptions);

        TestMarker();

    }



    // Function for adding a marker to the page.

    function addMarker(location) {

        marker = new google.maps.Marker({

            position: location,

            map: map

        });

    }



    // Testing the addMarker function

    function TestMarker() {

           CentralPark = new google.maps.LatLng(37.7699298, -122.4469157);

           addMarker(CentralPark);

    }

</script>