If you're a Plus or Pro member, you can view your Message Campaigns, in addition to edit or create new ones, by going to Email, Text, Voice > Message Campaigns. Just click on the pencil and paper icon next to any message campaign to view the email series inside. Click this same icon next to any of the emails inside a campaign to view or edit the email.
You can create new email campaigns in the middle section of this page (as shown in the screenshot below).
Once this is done, the new autoresponse campaign will appear in the list of Message Campaigns at the top of the page. Simply follow the instructions on editing/creating an email above to create your custom autoresponse campaign!