Rather than using `$(document).ready()` try this:

    <script>
    _spBodyOnLoadFunctionNames.push("ButtonFunc");

    function ButtonFunc() {
       $('.ms-qcb-leftzone').append('<li class="ms-qcb-item"><a href="http://www.google.com">ButtonName</a></li>');
    })
    </script>

If this doesn't work check that the selector `$('.ms-qcb-leftzone')` is actually finding something by doing something like:

    var elems = $('.ms-qcb-leftzone');
    console.log(elems.length);