Get question by faq-topic

I’ve installed the plugin “WordPress FAQ Manager”. It uses custom post types (question) and categories (faq-topic). I want to get the questions that belong to a specific faq-topic. I’ve tried get_posts and WP_Query and set category = $topicID, but that returns nothing.
I’ve googled and searched at this forum without result. This must have a simple solution and I hope I’m not the only one wating to do this.