Located on the editor tool bar is an icon < > that appears below (boxed in red).
Using HTML for Templates
Within the template editors located under the E-Marketing, Docs and Guidelines Tab there is the ability to create/edit using HTML coding. All coding must be done as in-line, meaning no header is used for styling, all style formatting must be done in line. Basically you will want all information needed to produce the final product as needed within the body of the HTML.
Below are some resources for learning more about HTML.