The first question would be to ask if you have updated to the new system in the archives yet. If you haven't, this would explain your problem right there.
If you have already upgraded, and are able to log in to the archive, you will notice that in the upper right corner, there is a clickable link that reads "My Control Panel." This is the new jumping off point to add or edit a story or chapter. When you click on that link, you are taken to a page that looks a lot like the archive Home Page, except that the menu column on the left side of the screen now has clickable links under each subdomain name. "Edit" will take you to a familiar dropdown menu page, with the choices that you've used before the new code, and this is where you can select your story to edit in that subdomain.