There should be a way to create a Nav menu item..
..that links to a Custom Post Type Archive page
i.e. what you get when you run:
get_post_type_archive_link('post_slug');
- - - - - - - - - - - - - - -
There is a plugin that does this, however it creates a static link (will break moving from development environment to live site..
http://wordpress.org/extend/plugins/cpt-archives-in-nav-menus/
Also, this guy is working on the issue, if his code is good it really should be added to core:
http://codeseekah.com/2012/03/01/custom-post-type-archives-in-wordpress-menus-2/
