MB Relationships – Connection Metadata?

Thank you! I have installed MB Relationships but am having trouble with the very first step. When I add the code to create a relationship (pasted below) to my functions file, it breaks the site. Am I missing something? I notice it says “Note: you need to hook to mb_relationships_init to make sure the API is ready for use.” — is that a separate step and if so do you have documentation for that piece?

add_action( 'mb_relationships_init', 'prefix_register_relationships' );
function prefix_register_relationships() {
    MB_Relationships_API::register( array(
        'id'   => 'posts_to_pages',
        'from' => 'post',
        'to'   => 'page',
    ) );
} );