This post is a bit old but just in case someone else (as I just did) goes hunting and hunting trying to find out how this works, the value for TagItemKey
needs to be set to BlogTagID. It seemed pretty clear that this was what you were asking for but never actually got an answer.
Hope this helps someone else out.
Now does anyone have the css to turn my cloud that currently displays as a list (with various font sizes) into an actual assorted cloud as you see on many blogs out there?