Adding videos is as simple as copying the embed code from the video provider (YouTube, Vimeo, etc) into your event description.
For example, to add this video to my event description, I simply add:
<iframe src='https://www.youtube.com/embed/botfLXowxVc' width='560' height='315' frameborder='0' allowfullscreen='allowfullscreen'></iframe>
…to my event description.
In this example, the embed code is directly from youtube, located underneath the video.
If you are having problems with the video displaying text instead of the video, replace all of your double quotes in the embed code to single quotes.