Updating to responsive: Can't get small menu to work

Used an older version of this great theme a few years back to build a site. Now trying to update the site and make use of responsive layout.

I heavily modified the theme back then, including tampering with template files, and didn’t create a child theme because it seemed like too much hassle at the time.

Unfortunately, this means I can’t just update the theme but now have to make the design responsive manually.

This is progressing ok with the styling – using the updated, responsive Portfolio Press CSS as a reference. But I simply can’t get the “small menu” to work.

I added this:
<h1 class=”menu-toggle”><?php _e( ‘Menu’, ‘portfolio-press’ ); ?></h1>
to the header, and the menu prints ok – but it doesn’t work. Nothing happens when clicking it.

Have spent hours and would be most grateful for a little help. How do I make this menu functional?

You can see the problem here: http://manifesta.dk/en/philosophy/

Thanks!!