Hi,
It sounds like you need to either:
* Drag the application playing the video onto the TV display
* Mirror the contents of your PC display onto the TV display by selecting the Windows key and the 'P' key and then selecting 'Duplicate' from the next menu.
If you've tried the above without success then something is wrong - please run through this guide and attach the logs:
http://www.displaylink.org/forum/showthread.php?t=896
Thanks,
James