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. Click on the pencil and paper icon to drill into the email campaign you've just created (which will appear in your list of email campaigns at the top of this page). Once you do this, you'll see there are no emails in that email campaign. Click the green "Add Message" button to create an email for the campaign. 

Follow the steps on the next page to create your email. Under Step 1, make sure you're saving your email to the correct email campaign (it should be automatically selected for you). Under Step 2, select how many days after someone subscribes to this email campaign you'd like them to receive this email (insert 0 to set an email to send out as soon as someone opts-in on a lead capture page). Under Step 3, insert your email subject line. Under Step 4, create your email (use the toolbar in this section to help format your email). When you're done, make sure to hit the green "Save" button at the bottom of the page. When you do this, you'll be brought back to the page to view the emails in that particular email campaign. You'll be able to view all emails in your campaign, edit and delete them.