where to add php code for paypal/terms checkbox??

[Jane1]

I am very happy to say I've found code to create a required terms and conditions checkbox for paypal: http://www.codingforums.com/showthread.php?p=1039354. However, I don't know which file to put the php code to make the checkbox function. Could someone look into this to find what I've missed?

Here are the highlights from the post:
Checkbox code to go inside the wordpress page's html tab:
<tr>
<td><input type="checkbox" name="chkTC" id="chkTC" value="yes" /></td>
<td><div align="left" class="maintext">I accept this site's Terms &amp; Conditions</div></td>
</tr>

Php code I need help placing:
if (isset($_POST['hdnSubmit'])) {

as well as this:

if (!isset($_POST['chkTC']) || $_POST['chkTC'] != 'yes') {
// error: terms and conditions was not accepted
die('You need to accept this site\'s Terms and Conditions to continue.');
}

I am halfway there and would love some help.

Thank you.