Edit in JSFiddle

function doSomething() {
    var list = document.getElementsByTagName("ul");
    var liTags = list[0].getElementsByTagName("li");
    var max = -1;
    for (var i = 0; i < liTags.length; i++) {
        var inp = liTags[i].getElementsByTagName("input");
        if (inp[0].value > max) {
            max = inp[0].value;
        }
    }
    var newLI = document.createElement("li");
    list[0].appendChild(newLI);
    newLI.innerHTML = '<input type="hidden" value="' + (parseInt(max, 10) + 1) + '" name="order">';
    alert(list[0].innerHTML);
}
doSomething();

<ul>
    <li><input type="hidden" value="7" name="order"></li>
    <li><input type="hidden" value="3" name="order"></li>
    <li><input type="hidden" value="6" name="order"></li>
    <li><input type="hidden" value="5" name="order"></li>
    <li><input type="hidden" value="1" name="order"></li>
    <li><input type="hidden" value="2" name="order"></li>
</ul>