Show MB Relationships field in Frontend Submission

Hello,

I have the Relationships:

 MB_Relationships_API::register( array( 
    'id' => 'opinion_to_local', 
    'from' => array( 
        'post_type' => 'opinion', 
        'meta_box' => array( 
            'title' => 'Locales', 
        ), 
    ), 
    'to' => array( 
        'post_type' => 'local', 
        'admin_column' => true, // show admin column, maybe needs paid extention? 
        'meta_box' => array( 
            'title' => 'Opiniones', 
        ), 
    ), 
) );

How can I show in the form Frontend Submission CPT 'opinion' the field to can choose the CPT 'local' ?

Thanks,
Sergio