Suggestions:
a. Look on the nltk directory installed on your PC. I checked mine and stanford.py is not there (i.e. is missing in nltk/tag/ directory). You can find quickly where to look for running this:
import distutils.sysconfig
print distutils.sysconfig.get_python_lib()+'/nltk/tag/'
b. If it's not there, then copy the stanford.py file from the source you mentioned to the nltk/tag directory on your PC (which you get in step a).
I hope it works out.
