Sorry, but add-ons/plug-ins aren't supported on Windows RT (desktop or metro IE). There is limited Flash support - so maybe Microsoft could build support into the browser as they did for Flash? That said, I don't know the future of Silverlight (there are rumors of it going away). Perhaps its future depends on the popularity of Html5. ??
Kate
It has a 10 year sunset of support ending in 2021, but it is pretty stable in Silverlight 5. The whole world is moving away from plugins like Flash and Silverlight. They were only created as a Band-Aid(tm) anyway to remedy missing functionality in most modern browsers back in "the day" (that was two weeks ago, right?).
Silverlight works just fine on the desktop version of IE10 in Windows 8.
The content providers who are providing streaming services will make a choice to move forward to HTML 5 any day now, so if your favorite SL site can't be rendered on your WinRT device, they'll modify it soon anyway to render on all devices unless they are crazy... :)
Cheers!
HAL_2010