var associated = $("select[name='associated_permissions']");
var associated_container = $("#available-permissions");

if (associated.val() == "custom")
    associated_container.removeClass('hidden');
else
    associated_container.addClass('hidden');

associated.change(function() {
    if ($(this).val() == "custom")
        associated_container.removeClass('hidden');
    else
        associated_container.addClass('hidden');
});