I forget my security question

Read the article you linked

You need to ask Apple to reset your security questions; ways of contacting them include clicking here and picking a method for your country, phoning AppleCare and asking for the Account Security team, and filling out and submitting this form.

You won't be able to add a rescue email address until you can answer your questions, from the HT5312 page that you posted from :

Note: The option to send an email to reset your security questions and answers will not be available if a rescue email address is not provided. You will need to contact iTunes Store support in order to do so.

You will need to contact iTunes Support / Apple in your country to get your questions reset, which you can do via one of the links that Niel gave e.g.http://support.apple.com/kb/HT5699 or http://support.apple.com/kb/HE57 or

https://ssl.apple.com/emea/support/itunes/contact.html