Well you saved the wrong thing on dropbox then. It is NOT the site folder that you need and that is no good because it is the published version of your site and iWeb CANNOT open that published version of your site, because it has NO import facility and is unable to open html and css files, which your published site consists of.
It is the domain.sites file that you need - this stores iWeb data and iWeb can open this. It is found under your User/Library/Application Support/iWeb/domain.sites. Be sure that you look in your User Library and NOT your System Library, as iWeb is not there. Also, in Mountain Lion, the Library folder is not visible, so you need to make it visible before you can get to it.
If you cannot find your domain.sites file - which you should have, unless it has been wiped from your machine, then check your Time Machine backup to see whether you saved your domain.sites file and if you did, then you are in luck and all you need to do is double click on it and iWeb will open it.
If for some reason you did not back up your domain.sites file and did not back up all your user applications, then you are out of luck and you will have to re-build your site from scratch in iWeb.
Remember next time to backup and backup the correct file too - it is your domain.sites file that you need and NOT a site folder - iWeb cannot open anything in a site folder because it is already published and iWeb has no import facility, so cannot open html and css files.