In your campaigns explanation it says "The third and final select box allows you to choose a user template, if any. Note that Gallery Templates are not supported; these must be User Templates. You can, however, copy a Gallery Template as a User Template and modify it to your liking."
What is the difference between a Gallery Template and a User Template? I checked on MailChimp's site and they don't seem to use these terms. Its quite confusing as to what you mean.