Doesn't show up in admin menu

Well, yep, this is the reason, it's not an admin section menu group nor node/add .
You can re-organize the menu structure here:
admin/build/menu-customize/navigation
So you can move multi_node_add menu to the admin section. Let me know if it helps.

This doesn't help if you are using the admin menu module: http://drupal.org/project/admin_menu

The link in the original post is broken - this should illustrate the problem: http://drupalcontrib.org/api/function/admin_menu_menu_alter
The admin menu module only adds items whose path begins 'admin' or 'node/add', regardless of whether they're in the administer section of the navigation menu. So the path 'multi_node_add' would have to become something like 'node/add/multi_node' or (probably safer) 'admin/multi_node_add'.

+1 to moving the URL to admin/multi_node_add . I'd very much like to use this module within the admin theme if at all possible.

You can still view the multi node add page with your admin theme, (or any other page) by setting a custom path with the admin theme module: http://drupal.org/project/admin_theme