Good news!!....take the "prescription" food back and get your money back!! It's got a guarantee....just tell them your cat won't eat it!
Then go to the store and buy some Fancy Feast Classics, Friskies pates or 9-Lives ground canned food....they're all under 10% carbs and that's what we want our sugarcats to eat!!
All dry foods are too high in carbs...including that expensive "diabetic" food the vet sold you.
What insulin are you using? The only insulins that are good for cats are Lantus, Levemir and ProZinc...and the good news is you don't have to limit your cat to only twice a day feeding!! It's actually easier on their pancreas to have several smaller meals instead of 2 big ones, but there are some harsher insulins like Vetsulin/Caninsulin that it's important that they get a bigger meal before the shot because when it "hits", it hits hard and can drop them too quickly if there's not a good amount of food on board.
Are you home testing yet? Home testing is very important because it lets you keep track of how your cat is responding as well as keeping him safe! It's also easier on your cat than dragging them off to the vet all the time for expensive and inaccurate "curves"...Cats respond to the stress of a vet visit by having their blood glucose go up to 200 points higher....the vet sees those high numbers, tells you to increase the dose and then you go home, your cat relaxes and you can end up with a tragic result.
It's not necessary to use an expensive "pet only" meter either. Most of us here use human meters. If you live in the US, the Relion Confirm or Micro meter at WalMart are both affordable and very popular among the members here.
I don't want to overwhelm you with too much information at once! There's lots to learn but the people here are all very good about sharing their real life experience in treating this disease and will help you along the way!
Do you have any questions?