Adding video content from youtube is very easy. Click the "insert" bubble in your "tool wheel" (or type "i" on your keyboard) and then click the "youtube" bubble.
This brings up a window where you can cut and paste the URL address for your video. Prezi gives you an example of how this link should appear, as shown in the below example.
Alternately, you can simply click anywhere in your workspace and paste the URL in the text box that appears; it will be replaced by the indicated video. See the video below for an illustration of both options.
To move or resize your media, see this page.
If you have a video file on your computer you wish to upload to your presentation, click the the "insert" bubble in your "tool wheel" (or type "i" on your keyboard) and then click the "load file" button as shown below. Alternately, press "L" on your keyboard.
This brings up a window where you can locate and select the appropriate media from your computer. The maximum upload size for files is 50MB per file. Supported video formats include: FLV, F4V, AVI, MOV, WMV, F4V, MPG, MPEG, MP4, M4V, 3GP.
To move or resize your video, see this page.