Sadly, those links are only for people who want to create
indie games. If you want to create avatar attachments, dashboard themes, gamer picture packs etc, then you can't do it through the XNA Game Studio path. You'll have to set up a contract with the Microsoft Xbox Live! group. Their contact information is available on this page:
http://www.xbox.com/en-us/dev/default.htm
You can find that link by going to
http://xna.com/ and clicking "professional developers."