<div class="modal fade" id="add-modal" tabindex="-1" role="dialog" aria-hidden="true">
    <form action="php_add/event.php" method="post">
        <div class="modal-body">
            <input type="checkbox" id="foodBox" class="flat-red foodBox" />
            <input name="food" id="food" disabled>
        </div>
    </form>
</div>

$(document).getElementById('foodBox').onchange = function() {
    document.getElementById('food').disabled = !this.checked;
    document.getElementById('food').value = "";
};