Just click on the Play button in the middle of the screen.
If you want to post a video proceed as follows:
1. Create a new post
2. Click YouTube button
3. Paste the YouTube URL link after the "[youtube]"
4. Click the YouTube button again
5. Click Preview and you should be able to view the video
6. 'Link' takes you direct to YouTube if you want
7. If it does not appear try removing the 's' from https so it reads http://