Custom Fields Code not echoing whats in the value field

I’m too new to be able to add a comment, but $post_id looks like it’s undefined. You’ll need to set it, one way is to grab it from the global $post object like so:

add_action('genesis_after_header', 'mbr_after_header');

function mbr_after_header() {
   global $post;

   if ( is_singular( 'post' ) )
      $cf = get_post_meta( $post->ID, 'mbr_customfield', true );

   if(empty($cf)) {
     echo "Empty Customfield";
   } else {
     echo "".$cf."";
   }
}