If you're using a coded email template and you're not seeing the preview text while editing your email, it's because your email template is missing a small amount of code.
To fix this, edit your coded email template to include the following piece of code as the first child element in the body tag:
<div id="preview_text" style="display:none;font-size:1px;color:;line-height:1px;max-height:0px;max-width:0px;opacity:0;overflow:hidden;"></div>
Click Publish changes to publish your changes to your template.