Well, it should work like this:
Go to your control panel (panel.dreamhost.com), to the Domains subsection, then Manage Domains. Find the domain in question in that list (it may be the only one), then click on "edit" for the Web line for that domain. That should take you to a page where you can set up the directory that the domain is attached to. It should have a line that says "Web" and lists something like this:
/home/[your user name]/[something you can edit]
Check what the [something you can edit] section says; it should be the name of a folder that is in the directory you see when you log into your account via a FTP server. That directory is where you need to put your index.html page to get it to work.
If the Web service shows Active, the directory is pointing to the right place, and that's where your index.html file is, then I have no idea what's wrong.