You don't seem to have the stopwords corpus on your computer.
You need to start the NLTK Downloader and download all the data you need.
Open a Python console and do the following:
>>> import nltk >>> nltk.download() showing info http://nltk.github.com/nltk_data/
In the GUI window that opens simply press the 'Download' button to download all corpora or go to the 'Corpora' tab and only download the ones you need/want.
~ Answered on 2014-11-01 22:26:03