1. Make sure you already have a form on your website. If there isn't one, you can add a form clicking Add and selecting form from the left tool bar.
2. Select the form you want to add an email address to. Then select the Cog from the settings menu that will have appeared.
3. Select the field Form Email,
4. Enter the email address you want into the Email field. Please ensure that click 'Finish' to save the email address.
Make sure you have added a submit button to your forms so that the completed forms can be sent to the email address that you have defined.