$newaddress = array( 'address' => '$newaddress' ); // Incorrect
Is not the same as:
$newaddress = array( 'address' => $newaddress ); // Correct
And you have a missing ;
at the end of:
$myrows = $wpdb->get_results( "SELECT address FROM wp_mediopay WHERE id = 1" )
And before the above line you must also have:
global $wpdb;