I see a lot of people struggling to link youtube videos to their serviio. I have created a chrome extension that should make the task trivial.
Simple how to:
Open the extension options and add your serviio host name and port number.
For videos just click on the add to play list button. If you don't have one already create a play list and add the video to the playlist. Then go to your manage playlist page and there should be an option to add the playlist to serviio. (remember to click refresh if you have made changes to the list)
You can also add playlists, search results and user feeds. Check out http://lazycowprojects.tumblr.com/serviiotube for more info and some screen shots.