You'll be able to to select plaintext as the format right before you send out your mailing. On the "Send Test" tab the drop-down menu labeled "Format" is located towards the bottom and is generally defaulted to HTML. On the Review and Send tab you'll see the same drop-down menu is located under the preview image of the mailing on the left.
If you need to make edit's to the plaintext version of your mailing you can do so by selecting the "Plaintext" mode of your mailing in the top right of the editor next to where "HTML" is highlighted.
Hope that helps! Let me know if you have any further questions :)