Brian and I e-mailed about this.
For future reference: This problem can't be solved. :-( WordPress.com, on their free hosted blogs, isn't allowing "iframes" at all. This means any widget that uses iframes to dynamically display content, like ours, won't work.
At some point we could think about developing widgets that use rss to display a feed. This would work for contributions widgets and we could do other neat things with G2G feeds and surname activity, e.g. there could be a widget that displays all Chelton activity on WikiTree. I can't think of a way to do the family tree widgets though.