Depending on the directions available at iWeb, you should be able to update your domain name servers to point to the iWeb host by following these instructions:
http://en.support.wordpress.com/domain-mapping/domain-management/#update-nameservers

Some sites, however, require that you use an A record to map your domain. If this is the case with iWeb, please let us know. Currently, we do not allow root level A records (i.e. for a root domain, such as example.com), but can let you know some other options. http://en.forums.wordpress.com/topic/wordpress-domain-to-point-to-a-iweb-site?replies=4