You can only add a rescue email address when you know the answers to your questions. If you don't know the answers then you will need to get them reset first. Try going to Express Lane and select 'iTunes' from the list of 'products' in the middle of the screen.
Then select 'iTunes Store', and on the next screen select 'Account Management'
Next choose 'iTunes Store Account Questions' or 'iTunes Store account security' (it appears to vary by country) and fill in that you'd like your security questions/answers reset.
You should get an email reply within about 24 hours (and check your Spam folder as well as your Inbox).
Or you could see if the second half of Kappy's reply in this thread helps : https://discussions.apple.com/message/20357006#20357006