Menu Trails doesn't work with Zen theme familly

Hello,

In my Zen sub-theme (Zen classic) I have activated the primary and the secondary menu.
But when I click on a secondary link, the primary link lost the active state.

So I have installed Menu Trail, and configured it.
But it seems not work out the box. Should I edit the template.php and add some lines?

The Menu trail's documentation don't speak about it, it seems to be so simple.

Thx a lot if you could share a solution.