Using AJAX in wp admin for plugin?

I’m a bit lost here, how do i run AJAX in the admin area of wordpress, my plugin basically needs to show a list of users in a tournament but i am not getting anything to change, when I select something from the dropdown nothing changes?

My main plugin file

function loadajax() {
	?>
	<script type="text/javascript">
function showPlayers(str)
{
if (str=="")
  {
  document.getElementById("txtHint").innerHTML="";
  return;
  }
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","<?php bloginfo('wpurl') ?>/wp-content/plugins/bowling-plus/getregisteredplayers.php?q="+str,true);
xmlhttp.send();
}
</script>
<?php
}

add_action('admin_head', 'loadajax' );

The file with the dropdown

<form>
 <?php
  global $wpdb;
  $table7 = $wpdb->prefix . "bowling_plus_tournaments";
  $req7 = "SELECT * FROM $table7 order by tour_name";
  $res7 = mysql_query($req7);

      echo '<select name="tournaments" onchange="showPlayers(this.value)">';
	  echo '<option>';
	  _e('Select a tournament:', 'Bowling_Plus');
	  echo '</option>';
      while ($data_ajax = mysql_fetch_array($res7)) {
          ?><option value="<?php echo $data_ajax['id'] ?>"><?php echo $data_ajax['tour_name'] ?></option>
          <?php

  }
?>
</select>
</form>
<br />
<div id="txtHint"><b><?php
	_e('Players will be listed here.', 'Bowling_Plus')
?></b></div>