Call the vet to find out what else you can do, I wouldn't give that to the cat at all. You might need to take the cat in to be seen. Hope this helps, good luck to you.
Actually, my older cat (19 years) is currently constipated, and after speaking to vet, was told to try giving the cat a teaspoon of cod liver oil. He said it could work as quickly as overnight. Also recommended: Mixing some canned pumpkin into the cat's food, to increase fiber. If these don't work, you do need to take the cat in to a vet, as the lack of bowel movements can essentially poison their systems and cause extreme illness and death. The vet will either administer a kitty enema, or they may have to anesthetize the cat and remove the blockage in another manner.
Not recommended, your best to feed your cat a moist cat food of a fish variety, one of good quality is better even if its in a smaller tin, something like high life tuna or salmon pate. They are naturally high in omega3 and fish oils. Directly giving human cod liver oil could give your cat runny poo!