When drafting a new email in an Email Marketing campaign, you'll notice that there are 3 different options in the top right corner:
Plain, which will send the end client a plain text email.
Pretty, which will send the end client an email with a header attached.
Designer, which will send the end client a nicely designed email.
In this scenario, click the "Designer" button to turn it into a Designer email. Click the "Add Designer Email" button in the middle of the page to open the Designer Email builder.
Once the builder is open, click the "Templates" button right above the title to choose one of your already created templates. Click "Overwrite this Email" top copy the template into your campaign email.
Once you're done, click the "Save: button to finish saving your email.