Basically, you DON'T get site files back into iWeb. As you have found, iWeb has no import facility, so is unable to open any html or css files - a published site and when you say site files, this denotes that you site is the published version that iWeb CAN DO NOTHING WITH.
iWeb stores all its data under your domain.sites file and this should save automatically as you are going along. It is this domain.sites file that you need to find and double click on it and iWeb will open it.
This domain.sites file can be found under your User/Library/Application Support/iWeb/domain.sites. Note that it is your User Library that you want and NOT your System Library, as iWeb is not under there.
I do not know if it is becasue I am in 10.7.5, but I cannot find it. When I look under Application Support, iWeb is not listed. I can do a search on "domain" and several sites are listed. I opened each site and they are all the older version.
I can then do a search for "sites" and I get a list of Folders and one of them has the current site we have up, in that folder. However, I cannot find the associated domain site.
I can go to my user file. When I click on the APPLICATION file nothing shows. I can go down the list to the SITES file and open it. The new site is in a folder inside the SITES folder, but once again, no associated domain site folder.
I used the program and it helped take me to SITES more easily, but it does not take me to the domain. I still cannot find the domain associated with the latest version of the web site. I can find the new "site" and the old "domain". I can open all the domains in iweb that show. Unfortunately, none of them are the new domain.
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
Once the library is visible go to your Users/Home/Library/Application Support/iWeb folder and look for the domain files there. You will need to use Roddy's applicaiton to select and open the domain file of your choice, however.