Having the perfect diet for gaining muscle isn't easy, being diabetic makes this more difficult since you'll also need to have your blood sugar under control. Altering your diet and exercise regime will require you to test regularly.
The basic nutritional guidelines for bodybuilding apply to diabetics as well. You'll need to have a caloric surplus in order to keep gaining muscles --> as you gain muscle, you'll gain weight thus need more calories. Between 1.5-2 grams of protein per kg of body weight is advised. Carbs become tricky as traditionally you'll need a-freaking-lot. Cutting some carbs and replacing them by fat (unsaturated) will make it easier to control your blood sugar but fat will never be as effective as carbs. But the better control you get from replacing a higher portion of carbs with fat pays off in the long run. The last few years i bulked with higher fat ratios and have had good results. But i do powerlifting and muscle gains don't really concern me that much, only strength. Though in a bulking phase (and still on a strength routine) i can gain about 4-5 kg of weight and according to my body composition scale about 1,5 kg of muscle. When eating carbs go for complex carbs and never simple carbs (a google search will yield lot's of lists). Just don't go overboard with carbs in a single serving as that will mess up your blood glucose.
Routine in your eating habits is also very important. I try to eat something every two hours, having a protein shake makes this easier as it's quick and easy. A good site to remember is
http://www.supplementlabtest.com/ as this will show you tests of most protein powders and their purity. You'll obviously need to use progressive loading on the resistance since that's basically how it works, your muscles get micro tears from resistance once they get "used to" the weight this becomes less and less so you need to increase the weight. Then your body rebuilds your muscle making them a bit stronger and bigger every time. This is where protein (and carbs) play a huge rule. Insulin also plays a major role in this body mechanic, therefor having a good control of your blood glucose post workout is important.
On protein powders: you can use them or not (they are definitely not mandatory for gaining muscle). You can get all you require from whole foods. But in the realm of biological value protein powders take the cake (the higher the better). It basically goes whey protein powders > milk > (lean) meats > fish > vegetables (as a rule of thumb). I use them a lot as i find them easy and unless you go overboard they're actually cheaper per gram of protein than whole foods. Proper whey shakes barely contain any carbs but stay away from weight gainers as they are just carb bombs. Personally i find the importance in nutrition to be (in descending order): macro nutrients (protein, carbs, fat), micro nutrients (minerals and vitamins), supplements. Timing of nutrition and hydration are also important factors.
If you want a good read on sports nutrition you can read Anita Bean's sport nutrition book, they're fairly straight forward.