Bruce, if you were not on insulin and losing weight, then went on insulin and started gaining, there's absolutely nothing wrong with the weight gain. At least, not in the "insulin made me gain weight" sort of "wrong." Before insulin, your body was unable to use and store the carbs you ate, and your blood sugar went up. Now, the insulin means your body can use and store carbs normally.
So you can't exactly blame it on the insulin (though some people feel that insulin itself can contribute to some increase in fat), but rather the increased normalcy of your metabolism.
It's annoying, huh? I was losing weight before diagnosis, then gained some after starting insulin treatment even though I'd started eating far fewer calories (while untreated, I had been hungry all the time). But despite gaining a little weight, I really do think I look healthier. To get rid of the weight/fat, there's really nothing to do but what the rest of the population does: eat healthier and get more physically active. Both of which can be tougher for us. :(