Did you go into the channel store to add YouTube? I have an XDS and use YouTube on it all the time.
I’ve looked all through the Channel Store and I simply can’t find the Youtube app. My Roku was purchased back in May and they have since come out with a Roku 2 series. Is that what you have? Perhaps that is the difference. If so, and they don’t make the app available to previous owners they’ve lost a customer that way as well.