Did you lose weight before you were diagnosed? This is very common as your body was breaking down protein (I think) as, without the insulin, it could not break down sugar.
Assuming you have your BG under control, this weight gain may be due to repairing the "damage" that was done before you were diagnosed.
My only other advice would be to ensure you are doing exercise in the "weight burning" region. An hour of weight lifting, for example, would not lead to weight loss.