Download and install NLTK from http://nltk.org/install.html if you do not already have it.
Move masc.py into your-path-to-nltk/corpus/reader/
In nltk/corpus/__init__.py add this code after the import statements,
masc = LazyCorpusLoader('oanc_masc', MascCorpusReader, r'(?!\.).*\.txt', encoding='utf-8')
In nltk/corpus/reader/__init__.py add this line,
from nltk.corpus.reader.masc import *
and add 'MascCorpusReader' (with single quotes) to the list of Corpus readers in the __all__ = [ statement.
Make sure you have the oanc_masc zip file and/or folder within your nltk_data/corpora directory.
The NLTK data folder is probably in your home folder. Inside of the MASC folder be the written and spoken directories.
You can download MASC 3.0.0 from http://anc.org but you need to rename the directory to oanc_masc and move it to nltk_data/corpora.
Open a python shell, such as IDLE,
import nltk
To use the MascCorpusReader type:
nltk.corpus.masc
For example, to see the list of files in the MASC directory type
nltk.corpus.masc.fileids()
For more example usage see http://nltk.sourceforge.net/corpus.html