The instructions on the page that you posted from should work :
- Navigate to appleid.apple.com using your web browser.
- Click "Manage your account"
- When prompted, sign in using your Apple ID and password.
- Click Password & Security
- You'll be asked to answer 2 of your 3 security questions before you can make any modifications. If you are unable to remember your answers, you can choose to send an email to your rescue email to reset your security questions.
- After you've validated your identity by correctly answering your security questions, click Edit to the right of your rescue email address.
- Once you've finished editing your rescue email address, click the Save button directly below the email field.
But if you don't know the answers to your security questions then you will need to get them reset first, and you will then be able to add a rescue email address for potential future use - in which case see if the instructions on this user tip for getting them reset helps : https://discussions.apple.com/docs/DOC-4551