From Bust A Tech:
To watch the video, simply select the video and press Enter or click it using your mouse. after the buffering complete the video should started. While the video is playing, you can always go back to choose another video by pressing Backspace, or click the back icon on top right of Windows Media Center. This is not a problem for those who are familiar with Windows Media Center. Beside browsing the videos through categories, you can also search for the video you like. Click on the search button at the video browsing page and type in the keyword to search.