Yes, iWeb 3.0.4 is the latest, version of iWeb 09, but you can ONLY download this update if you already have iWeb 09/3 installed on your Mac already. It will NOT work if you are trying to download this update and are trying to update from iWeb 08 to iWeb 09. As OT has already pointed out to you, it clearly states at the bottom of the update that you need to have iWeb 09/3 already installed on you computer or the update will not work.
If you have iWeb 08, then the only way to update to iWeb 09/3, is to go to Amazon and purchase either the iLife 09 or iLife 11 boxed set, both of which contain iWeb 09 and install this on your Mac and then you can download the updates if you need to.
Once again - read the small print underneath the updates and you will see that you need to have iWeb 3 already installed on your Mac to be able to download the updates.
Other than this, either you go to Amazon and purchase iLife 09 or 11 or at this stage it is probably better to update to a newer application such as EverWeb that is supported.