When you add a new campaign variation to any campaign, you have the option to use the visual editor. In order to use it, you have to choose one what type of interaction you want. In this case, you have to choose the “Display personalized HTML” option.
After selecting the type of interaction, a list of templates will show up and from here you can choose the one you want.
Once you chose the template you want to use, it will appear in the working space.