This is not the full source code, just the interesting bits for the wizard.

JavaScript:

	var isFormValid = function (evt, $link, $current, $target) {
		// checkbox in step 2 needs to be checked
		if ($current.attr("id") === "step2" && $target.attr("id") === "step3") {
			var checkbox = $current.find("input[type='checkbox']");
			if (checkbox.size() === 0 || checkbox.filter(":checked").size() > 0) {
				alert("OK, you can proceed.");
				return true;
			}
			alert("You really need to check the checkbox to advance.");
			return false;
		}
		// checkbox in step 3 needs to be checked, if not, redirect to step 1
		if ($current.attr("id") === "step3" && $target.attr("id") === "step4") {
			var checkbox = $current.find("input[type='checkbox']");
			if (checkbox.size() === 0 || checkbox.filter(":checked").size() > 0) {
				alert("OK, you can proceed.");
				return true;
			}
			alert("You are being redirected to step one.");

			// use the to method to change the active DIV
			$("#wizard-demo-3").wizard("to", $("#step1"));
			return false;
		}
	};

	$("#wizard-demo-3").wizard({
		onBeforeClickLink: isFormValid
	});

HTML:

<form>
  <div class='ui-corner-all ui-widget-content' id='wizard-demo-3'>
    <div id='step1' title='Step one'>
      <p>
	This demo is about the <tt>onBeforeClickLink</tt> event and the <tt>to</tt> method. On the next page you will find a checkbox.
      </p>
      <a class='wizard wizard-button' href='#step2'>&rarr; step two</a>
    </div>
    <div id='step2' title='Step two' style='display:none'>
      <p>
	<input type='checkbox' />You have to check this one! 
      </p>
      <a class='wizard wizard-button' href='#step3'>&rarr; step three</a>
    </div>
    <div id='step3' title='Step three' style='display:none'>
      <p>
	<input type='checkbox' />You have to check this one as well! If not, you'll be redirected to step one.
      </p>
      <a class='wizard wizard-button' href='#step4'>&rarr; step four</a>
    </div>
    <div id='step4' title='Step four' style='display:none'>
      <p>
	I guess you got the basic idea.
      </p>
    </div>
  </div>
</form>