This is a simple solution to put google maps in your admin panel. User can drag the marker and the position will be saved in latLang custom field:

piklist('field',array(
    'type'=>'html',
    'label' => 'Map',
    'value' => '<div id="project_map" style="height:500px;"></div>',

));

piklist('field',array(
    'type'=>'hidden',
    'field'=>'latLang'

));

?>
<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false"></script>
<script>

    var map;
    var marker
    var latLangField = jQuery('#_post_meta_latLang_0');
    var zoom = 15;
    var latLang = latLangField.val();    
    if(latLang == '') {
        latLang = "54.470636, 18.469391";
        zoom = 8;
    } 
    latLang = latLang.split(',');
    
    
    function initialize() {
        
        
        var mapOptions = {
            zoom:zoom,
            center: new google.maps.LatLng(latLang[0],latLang[1])
        }
        map = new google.maps.Map(document.getElementById('project_map'),mapOptions);
        
        marker = new google.maps.Marker({
            draggable: true,
            position: new google.maps.LatLng(latLang[0],latLang[1]), 
            map: map,
            title: "Your location"
        });
        
        google.maps.event.addListener(marker, 'dragend', function (event) {
            var lat = this.getPosition().lat();
            var lang = this.getPosition().lng();
            latLangField.val(lat+','+lang);
        });
        
    }
    google.maps.event.addDomListener(window, 'load', initialize);
    
   
    
</script>
<?php

You will have to change ‘#_post_meta_latLang_0’ to the ID of your hidden field.