You'll need to first add firstname.lastname@example.org to the Approved Personal Document Email List in your Amazon account. To do this, simply log in to www.amazon.com and visit the Manage Your Kindle page. On the left side of the page, look for and click on Personal Document Settings. Find Approved Personal Document Email List and click Add a new approved email address.
Once that is done, you can go to the Kindle setup page in Readability and put in your Kindle address.
*Amazon provides incoming email addresses for Kindle devices and the most recent iOS apps. You'll need the latest version of the Kindle app for iOS.