I don't have a wii so I can't really help you there.
But from what I read with a quick google, you need to convert the file to a format compatible with the wii and load it through the SD port. Not really worthwhile.
Personally, I got myself this:
http://www.wdc.com/fr/products/products.asp?driveid=572
Pretty neat and has been able to play my clips, along with some downloaded episode, even those at 1280p.