Hi,
While you can easily add the Yedda widgets to a WordPress blog hosted on your server, adding them to a blog hosted on wordpress.com is not very well supported at this point, since wordpress.com does not allow JavaScript.
A temporary solution would be to hack something with the WordPress Text widget and an IFrame, the best would be to create a native set of Yedda widgets for WordPress.
We'll get to it soon I hope (unless someone else will beat us to it - any takers?
).