I created a simple application using your code and it seems to work as expected, I have attached it here.
1. You say that you cannot set the ItemsSource of the MediaPlayer. Do you get an exception?
2. Can you confirm that you are generating MediaPlayer items (at least 1)?
3. Also please check that the video height / width that you set to the MediaItem is not 0.
Hopefully we will be able to get to the root of the issue,
the Telerik team