Every time you choose a template for a campaign, it will come with a button that automatically links to http://photobiz.com/email-marketing/. Of course, for your own campaign you'll want to update the link to where your customers will need to be directed.
In your campaign builder, click on the pencil and paper icon in the box containing the button. From there, click on advanced.
After you click on advanced, you'll be able to update the button text, button link, and color.
After the appropriate updates have been made, click on save changes.