In the email editor, click on the "Content" tab. Open the "Text" content type if it isn't already open. Click and drag a Paragraph, Title, or List area into a Row.
Presto! You've added a text type area.
Click within the text area to start editing and typing. On the right, you'll see the basics like font, size, and color as well as some more advanced customizations (line height, letter spacing, etc). In the WYSIWYG editor, which lives near the text area when you're editing, you'll see emphasis (bold, italics, etc) and link controls.