1.)   Add field to mysql table jos_ads, for instance URL
2.)   Edit administrator/components/com_adsman/tables/addsman.php
   Around line 28 add :  var $URL            = null;
   Have in mind PHP is case sensitive
3.)   edit templates/t_editadd.tpl
   add wherever you need the <input tag:
<input class="inputbox" id="url" type="text" name="URL" />
4.)   edit templates/t_details_add.tpl
   add wherever you need the tag {$add->URL}