I'm sorry to hear your cat is constipated. I would start treating by adding a small amount of vegetable or mineral oil to the feedings. Just a few drops at first, as you don't want to overdo it and cause diarrhea. You can always increase the amount a bit. The oil will lubricate the intestinal lining and push the contents along the tract more quickly than normal.
Once regulated, stop the oil, but you may want to add a higher fiber to the diet, like regular canned pumpkin or cottage cheese when starting to see firmer stools, or you can even compare what you are feeding now with higher fiber diets sold in the store (fiber content listed on packages). Metamucil can also be used if ok'ed by your vet at 1g per pound of weight once to twice a day as needed to maintain stool consistency. It does not work overly well when used for an already blocked up cat, but if the situation hasn't become immediate yet, you may be able to try this first.
If the problem becomes chronic regardless, there could be an underlying medical reason on why the intestines are not absorbing appropriate amounts of water, and then a vet visit with some blood work may be in order. If you ever see increased urinating along with constipation, that would be a key sign that your cat is getting dehydrated and needs vet assistance.
Best of luck, and let me know if I can help further. Hope your kitty is better in no time.