Unfortunately, although what Tom says is true, there are so many examples of using minerr in the Collaboratory that finding a good, easy to understand one, is not necessarily easy. You can find a simple example if you go to "help", quicksheets", "Data Anlaysis", "Using minerr for nonlinear least squares fitting".
If you can't get it to work after reading that, post a worksheet containing the data and the function you want to fit.
Richard