how do i set up a rescue email address on an existing account?

You won't be able to add one until you can answer 2 of your 3 security questions, so you will need to get them reset first - see if the instructions on this user tip helps : https://discussions.apple.com/docs/DOC-4551

When you've got them reset you can then add a rescue email address as described on the page that you posted from for potential future use :

1. Navigate to appleid.apple.com using your web browser.
2. Click "Manage your account"
3. When prompted, sign in using your Apple ID and password.
4. Click Password & Security
5. 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.
6. After you've validated your identity by correctly answering your security questions, click Editto the right of your rescue email address.
7. Once you've finished editing your rescue email address, click the Savebutton directly below the email field.