There are no sliding menus on the support page mentioned earlier. Those are accordion drop down menus. js is used.
A sliding menu can be found
Here at the top of this page. (wi-fi,accessories,Macs,Applications,servers)
Both accordion and sliding menus are not something that can just be inserted into an html snippet and work it's more complicated than that. First off you need to add the js reference to the head section of your html file, something an iWeb html snippet is incapable of doing. Furthermore you have to upload referenced js file to your host. Also the js file that functions for these menus conflicts with js file generated when you insert an html snippet into iWeb, so using an html snippet will not work anyway! These are just some examples explaining to you that you may be getting in over your head with this one! It's involved and complicated and certain knowledge is required for functionality. Something not meant for iWeb.
If you get it to work in iWeb kudos to you!