This may be something that MFP adds in the future. But if your Dr. has told you how many grams of carbs you are allowed for meals and snacks, you can adjust the percentages accordingly. I have my macros set to 30-30-40 (30% carbs, 30% protein and 40% fat) and sugars at 24g.

@scrubman, one of the frustrating things about Diabetes is that it is a very individual complaint. What works for one person, may not work for the next. It really is a case of working out what works for you as an individual and then sticking to that plan like glue. Having said that, there are some generally accepted rule of thumb approaches which are applicable to most people. Sticking to complex carbs rather than simple carbs (google this if you're not sure). A consensus view that a reasonable target to aim for would be 45 net carbs per main meal with say 20 net carbs for snacks if required. If you're new to Diabetes then I suggest that you try for a place on a Diabetes education course. The vast majority of people who attend these courses find them to be of great benefit.
Ref your specific point about MFP and Diabetes. The programme is designed to help people adopt a healthy lifestyle and diet. It has proved to be an absolutely fantastic tool for those of us with Diabetes. While not specificaly geared for Diabetics it is simply a case of adjusting the net carb targets downwards to meet the targets suggested above. Perhaps the greatest benefit for Diabetics is the support, encouragement and exchange of information which this support group provides. Take time to read over the various threads and get a feel for the site. When you're ready to ask questions you will find lots of support and encouragement on here. I wish you well on your journey to a healthy and active life and will certainly help in any way you think may be useful to you.