Can I use a quote from Seinfeld in a magazine ad?

While quotes are not protected by copyright law (at least if they are short), they may be protected by trademark law. But your real problem here is that you would be violating Seinfeld's right of publicity--using his name or a quote associated with him to market your product without permission from Seinfeld. Anytime you try to benefit by associating yourself with a famous celebrity or product (or television show), you are engaging in illegal conduct if you do so without permission of the owners of the applicable intellectual property rights. If you are involved in creating magazine ads, then you need to educate yourself about the basics of intellectual property law. And even then you need to retain and work with intellectual property counsel to advise you on matters such as this. Frankly, anyone who is successful in the advertising and publishing business has intellectual property counsel on speed dial.