Collapsing sub-menus with CSS

Hi,

I want to create a custom menu with sub-menus that collapse and expand. I found this post describing the topic:

http://wordpress.org/support/topic/create-collapsing-sub-menus-with-css-wordpress-30?replies=8

There is also an example of CSS implemented in the following link. You click ‘Sample Page’ and the sub-menu expands. You then click ‘subpage two’ and is stays expanded. When you click any other menu-point it collapses:

http://demo.madebyraygun.com/portfolio/sample-page/subpage-two/

The above is exactly how I want the menu to function on my webpage. But when I click a sub-menu-point, it shows the content of that page/category/whatever, but the menu collapses instead of staying expanded.

http://entasis.dk/wordpress/category/projekter-projects

When you click any sub-menu-point the menu collapses.

What am I doing wrong?

best,

Troels