Hello Franck Casado,
For the moment RadMediaPlayer, doesn't support Server side playlists, including ISX files. The best you can do to use server side playlist is to handle the playlist(call the web content through the standard silverlight methods) in your silverlight app. and convert the result to RadMediaItems. In other words to create MediaItem (client side playlist item) for-each server side playlist item.
If you need further assistance, please contact us.
the Telerik team
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