You should make sure to get the login part worked out first. If you are using Security Assist on your site for user authentication then you will want to apply a Security Assist access rule to the page and not the standard DW one.
You can do this from the Modify > Security Assist > Access Page Manager.
The error you are getting about the selection and the body just means that you need your cursor to be within the <body> and </body> tags on the page when you are trying to add in this server behavior.