iWeb was intended to allow people to put up a decent website without knowing or seeing any HTML code. So it really does not have an HTML editor. It does generate HTML code. That means you can add HTML code to iWeb generated pages, but the actual insertion of the code cannot happen until the iWeb site is published, causing iWeb to generate the actual pages with HTML code.
If you publish to a folder, you can just go in and edit the code to add your HTML. A useful suggestion I found at Rowan Cottage is to create a text box with an identifiable string as a placeholder so you can easily find the spot you wanted to add your code.
There are also tools that allow you to enter the code in iWeb and then post-process your iWeb site (run after you publish the site) so that you can enter the HTML code in a rounded box within iWeb and not have to worry about putting the code manually into the generated code.
iWeb Enhancer is $12.95 and iWebMore is free.
I’ve played around with iWeb a bit but I mostly use Dreamweaver to build web pages, so I haven’t tried any of this except the trick of putting a place holder in a text box and finding it in the generated HTML code.