Where?

I've just bought the <h3 class="t-heading -size-xs" data-analytics-element-id="item link">Wpcomomatic WordPress.com To WordPress Automatic Cross-Poster Plugin for WordPress</h3> to post from Wordpress.com to my Wordpress.org site but for the life of me I cannot remember which site I install it on.   Does it go on the site that I want archived (wordpress.com) to send to the archive site (wordpress.org)? Or Do I install it on the wordpress.org site to call the posts FROM wordpress.com?