Hi,
That sounds like you're not logged in properly. When you first create or activate an archive account, you use the center fields to log in, but after that point, you need to use only the fields in the upper right corner.
Go up to the upper right corner of the archive page, and log out, even if you think you're already logged out. The fields should revert to blank. Then input your email address and the password in those upper right fields. Remember that passwords are case sensitive, so if you used a capital letter anywhere when making the password, it always has to be a capital letter.
Once you go to Control Panel > Harry Potter > Add, you should see a screen with a number of fields:
The rest is pretty self explanatory. You complete the fields, enter the recaptcha, and get taken to the Rich Text Editor where you can add the first chapter of your story.
Adding additional chapters will bypass this screen and take you straight to the Rich Text Editor, so that part is easier.
I hope this helps!