Select the YouTube video option from the list of Blocks and you will be asked for a Title, the URL of the YouTube video, width, height and a video player option.

Title - Give your video a title, this will be displayed on the page so is optional.
YouTube URL - Paste the URL of the YouTube video here, you must have already uploaded the video to YouTube for this option to work.
Width/Height - Define the height and width of the video here
Video Player - You can choose either option here, its best to go with the iFrame option at present as Apple Mobile devices don't allow flash.