To Activate eNewsletter Sign-Up on a Form:
  1. Navigate to Communities > Forms > [The  Type of Form On Which You're Working]
  2. Hover over the form you are editing and click Website Info
  3. Click on Form customization
  4. Click on Add-Ons
  5. Under eNewsletter Sign-Up, mark the Active radio button next to eNewsletter of your choice
  6. Then go back to the website info of the form and click on publish this event now