The Domain Restore feature is found in the Manage Domains section. Each domain listed against your account has a restore button on the right hand side. Click this to see the details of the feature. Don't worry, it won't do anything to your domain.
Do note though that DreamHost make no guarantees at all for actually backing up your data so personally I wouldn't rely on this. It is strongly recommended that you backup all of your web and email to another site. You can also use the backup user too but again, I don't think DH provide any service agreement for this data. If you are backing up for yourself, don't forget to regularly dump and backup your SQL databases too.
You seem to get very little control over what has been backed-up or when so restoring is also somewhat hit-and-miss.
In most cases, if the automatic restore fails we are able to try a manual restore. Let Dreamhost support know the date you're looking to restore and they will be happy to see what they can do.