publish_post doesn't work for custom post types, the correct hook (action hook) is publish_{$custom_post_type}. You should use add_action() as this is an action hook.
I also tend to make use of the transition_post_status hook which is a much more universal hook as it fires everytime a post's status is changed regardless. You can use $old_status and $new_status to check the previous and new status of a post then do something.
For a new post, you can something like this: (Requires PHP 5.3+)
add_action( 'transition_post_status', function ( $new_status, $old_status, $post )
{
if( 'publish' == $new_status && 'publish' != $old_status && $post->post_type == 'my_post_type' ) {
//DO SOMETHING IF NEW POST IN POST TYPE IS PUBLISHED
}
}, 10, 3 );