error in ajax function

hi
when i run the following code i get an error of:

$("#addform").ajaxForm is not a function. because i am only just learning this language could someone show me why this is causing error. is it because i am not calling the form id directly but the div? many thanks

<script type="text/javascript">
function add() {
// prepare the form when the DOM is ready 
$(document).ready(function() { 
    // bind form using ajaxForm 
    $('#addform').ajaxForm({ 
        // target identifies the element(s) to update with the server response 
        target: '#htmlExampleTarget', 
 
        // success identifies the function to invoke when the server response 
        // has been received; here we apply a fade-in effect to the new content 
        success: function() { 
            $('#htmlExampleTarget').fadeIn('slow'); 
        } 
		
    }); 
});}

form
=================================================

<div id="addform" style="display:none;">
  <form action="filesadd.php" method="post" class="webform">
    <fieldset>
    <legend><span>Enter a File</span></legend>
    <label for="box">Select a Box</label>
    <select name="box">
      <option SELECTED VALUE="">Select a Box</option>
      <?php
do {  
?>
      <option value="<?php echo $row_Recordset1['boxref']?>"><?php echo $row_Recordset1['boxref']?></option>
      <?php
} while ($row_Recordset1 = mysql_fetch_assoc($Recordset1));
  $rows = mysql_num_rows($Recordset1);
  if($rows > 0) {
      mysql_data_seek($Recordset1, 0);
	  $row_Recordset1 = mysql_fetch_assoc($Recordset1);
  }
?>
      
    </select>
    <label for="fileno">File Number:</label>
    <input id="fileno" name="fileno" class="text" type="text" />
	<div id="htmlExampleTarget"></div>
    <label for="authorised">Authorised By:</label>
    <input name="authorised" type="text" class="text" id="authorised" value="<?php echo $_SESSION['kt_name_usr']; ?>" />
    </fieldset>
    <input class="submit" type="submit"  name="submit" value="Add File" />
  </form>
</div>

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