If the function you are referring (myfun) is defined in a script, then it is going to be a local function and cannot be accessed outside the script. In that case, as dpb pointed out you may have to save myfun as an m-file and add it to the MATLAB path.
Also, make sure you are passing the argument as function handle in nlintfit function. Please find the documentation for the nlintfit function. If you would like to know more about the function handle, you can find here.