inner jQuery won’t work

Try this:

jQuery(document).ready(function() {
    jQuery('#showmenu').click(function() {
            jQuery('.menu').slideToggle("fast");
    });
});

WordPress hates the $ statement. If you replace it with jQuery, it should work.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)