Untitled

<html>
<head>
<script type="text/javascript" src="./script/jquery-1.2.6.min.js"></script>
</head>
<body>
<script type="text/javascript" language="javascript">
$(document).ready(function(){

$("#donation_selector > input[@type='radio']").change( function() {
  var id = $(this).attr('id');
  if ( id  == 'custom_ammount') {
  $("#custom_ammount").attr('disabled', false)
}


})
});
</script>
<form id="webform-client-form-3">
<div id="donation_selector">
<label><input type="radio" name="donation" value="25" /> $25.00</label>

<br />

<label><input type="radio" name="donation" value="50" /> $50.00</label>

<br />

<label><input type="radio" name="donation" value="100" /> $100.00</label>

<br />

<input type="radio" name="donation" value="custom" id="custom_amount" /> <input type="text" name="custom_donation" disabled="true" />

<br />
</div>

</form>