The main ingredients to a Paleo lifestyle are plants, animals, nuts, and seeds. In that order. Flax seeds can definitely be included as part of a Paleo lifestyle, just remember how much work they require to harvest, so a hunter-gatherer would not be eating a lot of them. In moderation, though, these little seeds can definitely be beneficial.
Not only are flax seeds low in carbohydrates, but they are loaded with fat and protein, a whopping 42% fat and 18% protein. Better yet, most of that fat is healthy omega-3 polyunsaturated fat. Omega-3 fats are anti-inflammatory and necessary for both our brains and our hearts to be healthy.
In addition to the impressive macronutrient content, flax seeds also come with an array of micronutrients, including calcium, magnesium, and potassium.
Mark Sisson includes flax in his list of edible seeds here.