I have added this on my site:

It appears right below the "Allow online applications" checkbox.

I will try my best to explain what I did...

1.) In "publish-write.tpl" look for your form line: (<form id="publish_form" method="post" action="  {$BASE_URL}post/")

Change the form line to: (<form id="publish_form" method="post" action="{$BASE_URL}post/" onsubmit="return checkCheckBox(this)">)

2.)In the same file go down untill you find your submit line (<fieldset><input type="submit" name="submit" id="submit" value="{$translations.publish.step1_submit}" /></fieldset>)

directly ABOVE the submit line add the following:

<fieldset>
<input type="checkbox" value="0" name="agree"><b>I agree</b> to the <a href="Yourtermslocation" target="_blank">terms and conditions</a> (must check to continue).</fieldset>

3.)Create a new file called acceptTerms.js (yoursite/js/acceptTerms.js)

add the following to acceptTerms.js:

function checkCheckBox(f) {
  if (f.agree.checked == false) {
    alert('You must agree to our terms and conditions to continue.');
    return false;
  } else
    return true;
}

4.)In "header.tpl" be sure to add this line accordingly:

<script type="text/javascript" src="{$BASE_URL}js/acceptTerms.js"></script>

Doesn't take long and it dishes out an alert pop-up if they don't check the box.

To check mine out: http://www.wpgjobs.ca/post