You will then have a window open in front of you with various options to choose from. You want to highlight the Identity selection under the Mail & Groups option to open the screen shown below.
Note - If the Mail & Groups option has a "+" plus sign in front of it, click on it to open the sub-menus.
Fill in Your name and your E-Mail and Reply-To address where the example shows visitor
(the use of visitor here is an example, you will want to use you own Surfari username and password in the appropriate places)