Adding new taxonomy to WP menu gives error

Ok, this is the first time a created something with this plugin, so I’m sure I must have forgotten something.

I created a new custom post type (products), then a custom taxonomy (items) and added a few categories to the items: games, cars, flowers.

Then I added some new products, chose game or car or flowers, and then I wanted to add the taxonomy ‘items’ to my WP menu. But I get a message that it is not valid, and the menu is not saved.

What am I doing wrong?

http://wordpress.org/extend/plugins/wck-custom-fields-and-custom-post-types-creator/