I am having trouble getting the simulations to work for a particular problem and was wondering if anybody has already looked into this?

The problem is that we have got a sample size based on a difference in means. We are going to use the negative binomial distribution to look at the difference between the number of events over a period of 2 years. One of the independent committee members thinks that the power we will have will be much smaller than we think (based on the difference in means)

I was wondering if anybody has done something similar with powersim before and if so could you give me a pointer?