Honestly, I would not recommend learning to teach through a DVD. If I was to take a Zumba class, I would expect that the instructor would be properly trained to teach it.
Would you want to take a yoga class from an instructor who learned from a DVD? Would you pay the same money to someone who just works out at a gym as you would for a certified personal trainer? I think if you want to teach any classes and get paid, then you owe it to the participants in your classes, as well as your employers, to get the proper training.
If you are already a fitness instructor, they probably offer a basic workshop geared towards fitness instructors who want to add Zumba to their portfolio.
Elle