I have never used the javascript site builder but I would be happy to try and help you. Perhaps you could email me some screen shots of the different menu options available in the backend.
p.s. is there an option to view HTML or Source code?
There is no option to view the source code that I have yet found. It does give me the option to bring up a form for each page and insert tags into the header, body and post-body. That's the closest thing to editing the actual html I have found.
try posting it, go ahead and submit outside of preview - view. Perhaps the preview view does not load the javascript. In any event copy and paste 'what you see before you paste in the rss code' into a notepad document so that you can restore if needed.
The code should work after the <body> tage but before the </body> tag.
Another thing would be to find out if yahoo site builder allows cut and paste javascript, for whatever reason they may not allow it.