Follow these steps

1. Whenever you want to display a video inside a content element, you need to select a dummy file with the file ending “.rgg” as a placeholder. This file can be empty (fileadmin/DEMO/dummy.rgg) (mandatory!).
2. Insert the Position
3. Insert the Image Dimension (mandatory!)
4. Insert a caption - i.e. copy the description from youtube
5. Add the URL to the video in the “Alternative Text” field using the following syntax: file:URL  i.e.  file:http://www.youtube.com/watch?v=UUH3tZuDrCY
6. Save and Test
7. Attention: Please insert always one dummy.rgg file each, per Video or Multimedia Element, if you want to show more than one. 
   i.e. file:http://www.youtube.com/watch?v=7T406i8lljE Attention: Start for each Multimedia Element a new line!