I'm very excited about this new project and can only imagine the possibility for educators to post little mini quizzes with their videos. The thing to remember is that this is only a beta and YouTube can take away this feature at any time.
First you will need to opt-in for the beta. Go to www.youtube.com/video_questions_beta . After you have uploaded your video, click on the edit button above your video. In the top bar you will see a questions button. Clicking on that will show where you can post some multiple choice questions at different time stops to your video.
Remember this is only for videos that you have created and that, for now, it is only a beta and can be disabled by YouTube at any time.