Cannot add any forum

I am trying to add a new category, save it.
It says: “category added”
but I don’t see any category!
Then I try also to add a forum, but I cannot!

What am I doing wrong?
Here’s my forum page:

http://lnx.sinapsi.org/wordpress/forum/domande/

Please help!

http://wordpress.org/extend/plugins/mingle-forum/