You must set a username in order to edit pages.
If you already have an account and you have set a password, you can login with your numerical user ID.
Hint: If you don't remember your numerical user ID, you can find it by looking for one of your changes in the log of recent changes: just point to your username next to the change log entry with your mouse, and your user ID will be displayed as a tool-tip or in the browser's status bar.