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The only way I can think of is to install PHPBB (or forum software of choice) to your webserver and then link to that from your main site.

Having said that, I've never used iWeb before (and never plan on doing so).
 
If you are using .Mac, you can't do this..

If you are using iWeb to make your site, I'm guessing you don't really have the technical skills yet to run a PHP/MySQL based forum..try something like this http://www.freeforums.org/
 
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