I had to lose weight as well. At our center, they were looking at BMI. The surgeon told me that not only were they concerned about the health risks of extra weight, but it also added complications to the surgery in obtaining the kidney. They recommended Weight Watchers, which was not a bad recommendation to follow. I actually ended up slightly over the BMI they wanted (30), but after the donor doctor examination, she said my weight was fine and I was eligible for donation.
It is a commitment and hard to lose the weight, but it will certainly be a long term benefit.
Best of luck!