I'm not sure what you're even trying to do. What is your app? How is it loading and is it loading the Joomla Framework and Joomla CMS already?
Our Sendy plugins and Sendy library are obviously meant to work within the Joomla CMS. In general, our System plugin is what loads the Sendy library and it does so with the following line:
Even more simplified though, you can directly require our Sendy class:
require JPATH_SITE . '/libraries/sendy/sendy.php';
That should include our class, but again, I'm not really sure how your App is working or what is different about it and the normal CMS.
Thanks,
Alex