This guide will help you configure the PlainHTML plugin correctly, this includes:
You have to select a namespace for the content of this plugin. This namespace may NOT be in use currently, if it's you registration will fail. Pages created with this plugin will be publicly available under the namespace you select. If you choose the namespace: MyNameSpace, and create a page named: MyFirstPlainHTMLPage, you may access the page publicly at this address: CMS3URLMyNameSpace/MyFirstPlainHTMLPage. Namespace:
You have to select a default page this plugin must show when it's namespace is accessed without any page title. If you don't specify one the request will return a 404. Default page: OPTIONS
If you have pressed the apply button during each step, you should be done configuration this plugin.