There is an example project you can use: https://github.com/EvanDotPro/EdpForum
Basically, the service can be obtained with:
$discussService = $this->getServiceLocator()->get('edpdiscuss_discuss_service');
After that, you can use methods of the service by exploring its public interface:
https://github.com/EvanDotPro/EdpDiscuss/blob/master/src/EdpDiscuss/Service/Discuss.php