Responsive DIY Forms was added in the 7.12.0 Release on 9/14/2014.  If you haven't already, you will need to Go Live with the form again to publish this enhancement.
  1. Click Management on the menu bar
  2. Click DIY Forms
  3. Click Edit next to the form you are working with
  4. Click Go Live in the upper right corner
If you are using an embeddable layout, you will also need to re-embed the form on the page your organization is hosting.