I'm overweight and out of shape (although I have been doing walking videos for over a month now) and can't do any belly exercises whatsoever,but I can do arm and leg ones with no problem.Should I keep doing what I'm doing now,or should I stop doing Pilates and take it up again when I'm more fit?

What do you mean you can't do any belly exercises? Have you had abdominal surgery? Or do you mean you don't like your level of ability to do belly exercises. Unless you have a medical reason, do not quit your Pilates. Even if you're at the stage where you just tighten your muscles without any other movement, you're getting stronger and fitter, and Pilates is a great mind-body exercise , as well.