Click the Newsletters tab on the left hand side of the Control Panel.
Either click the ADD A NEW NEWSLETTER link on the left of the site, or click the big "Add a new newsletter" button on the top right.
Click on the "Advanced" tab at the top of the page, and then click "Import from URL".
A popup will appear asking you to give your newsletter a friendly name, along with a "Website URL" text field. Enter the URL of the web site or web page that you would like to import (Including the HTTP:// or HTTPS://).
Click the IMPORT FROM WEBSITE button and your HTML will import.
You will be taken to the HTML Editor with your uploaded HTML loaded up, where you can make any last minute changes to your newsletter.