“Celery seed” is another misnomer. The seeds referred to are actually fruit, and fruits are certainly on the Paleo menu.
Despite their fruit status, celery seeds are often ground up and used as a spice to flavor food. Celery seeds are often used to provide the celery flavor in celery salt. You can usually buy it anywhere you buy your other spices and add to soups, stews, stir-fries, or salads. Feel free to play around with it to find out which dishes you think are better off with a little celery seed.
Celery seeds have been used for millennia as medicine to relieve pain and they have been shown to lower blood pressure in rats. Like all spices, celery seed can provide a significant source of immune-boosting anti-oxidants. For more information on how to choose your spices, check out this post.