The code above seems pretty much copied from the example at http://piranhacms.org/docs/extend/manager-menu. If you take a look at the last paragraph in the documentation you can see that in the above example two custom permissions have been added for the new manager functions (ADMIN_PRODUCT, ADMIN_PRODUCTGROUP).
Since the manager only renders items accessible by the logged in user I can only assume that you haven't added these permissions under System > Permissions which results in the current user not have access to them. And since the user hasn't access to any items in the group, the entire group is hidden!
Just remove the Permission = "..."
from you items and they should render.
Regards
Håkan