I have created a test site (https://sites.google.com/site/danielstestpagina/) where I have added the widget more or less:
Edit the page where you want to add the widget
Select Insert -> More Gadget
Search for 'iframe'
Select the Include Gadget (iframe) by Scott Johnston
It is not possible to paste the whole widget code into a box but when you add the widget URL (http://www.wikitree.com/treewidget/Sample-7/ for the demo widget; look for the part right after src=" in the second widget code line) and set the frame size to a width of 550 and the height to 520 it should display properly!
p.s. You need to set the widget title yourself and there is a length limit.