I think currently, there is only the 32 bit version of flash player. In that case, 64 bit Internet Explorer can't use it.
But Adobe is rolling out a 64 bit version of flash player so pretty soon 64 bit internet explorer can use it.
I think it's not out yet (unless someone posts me a link verifying i'm wrong).
That should be the reason why you can't see flash videos on 64 bit IE even though you have flash player installed