Hi Laurie and Melvin and welcome to the forum.
If Melvin is not eating he may be nauseated. Some cats when first diagnosed do not want to eat much. But it is really important that he eats. If he doesn't start eating by tomorrow I would go back to the vet and ask for some cerenia tablets for nausea.
Do you know if the vet tested for ketones and if Melvin had any? Some cats when diagnosed with diabetes can have ketones and as they can progress into DKA which is a very serious illness,the ketones need to be acted on. So it is really important that Melvin is eating, drinking and is getting the prescribed insulin.
Did Melvin have an infection as well or just the diabetes?
Can you tell us the type of insulin you are giving please?
Here is a link to food charts. Choose the one for the country you live in. The top 4 are for the US.
http://www.felinediabetes.com/FDMB/threads/links-to-food-charts.174182/
Diabetic cats need to be eating a wet/canned food diet with 10% carbohydrates or less. Have a look in the charts and choose ones that are 10% or less. A lot of people use Fancy Feast. There is no need to buy the expensive prescription diets from the vet. They are no better than the foods you can buy in the supermarket and pet stores. Dry food is too high in carbs.
Here is a link to FAQ which you may find helpful.
http://www.felinediabetes.com/FDMB/forums/health-links-faqs-about-feline-diabetes.14/
Have a look at the yellow stickies at the top of the FAQ page about hypos and how to deal with them. Make sure you have some high carb food (over 16%) in the cupboard and some honey or Karo in case of low numbers. We suggest printing the hypo pages and putting them on the frig, as when you get a low number, you don't want to have to look for what to do,much easier to have it on the frig and the highcarb and the honey in the cupboard.
It is a steep learning curve in the beginning but it gets much easier.
This is a very supportive community and we are happy to help on any way we can.
You could try sprinkling some Parmesan cheese on top of Melvin food to tempt him. Here is a link to stimulating kitty's appetite
http://www.felinediabetes.com/FDMB/threads/suggestions-on-how-to-stimulate-kittys-appetite.130770/
Looking forward to seeing you again. You may like to start posting over on the main health page where there are many more people to help and support you. Here is the link.
http://www.felinediabetes.com/FDMB/forums/feline-health-the-main-forum.28/