In PYTHon3, it runs IDLE:
import nltk
nltk.download()
Tip: Certificate Verity Failed

Solutions:
Run in TERMINAL:
/ Applications/Python 3.6/Install Certificates.com mand

Then run NLTk.download () in IDLE.

For details, please see: https://stackoverflow.com/questions/41348621/ssl-error-downloading-nltk-data