Sorry their bbcodes are a little be tricky here !
Open jm-twitter-cards.php, add this line 58 :
//we add a new post meta to our post
$creator = get_post_meta(get_the_ID(),'twitterCreator',true);
then add this line 71:
<?php //this is the markup for new field ?>
<p>
<label for="twitterCreator"><?php _e('Enter your Personal Twitter account', 'jm-tc'); ?> :</label>
@<input id="twitterCreator" type="text" name="twitterCreator" class="regular-text" value="<?php echo esc_attr($creator); ?>" />
</p>
and last replace code on line 96 to 98 with this code :
// here you update value
if( isset( $_POST['twitterCardType'] , $_POST['twitterCreator'] ) )
update_post_meta( $post_id, 'twitterCardType', esc_attr( $_POST['twitterCardType'] ) );
update_post_meta( $post_id, 'twitterCreator', esc_attr( $_POST['twitterCreator'] ) );
}