crosstab function

I run Postgres 10.5.  I understand that there is something called tablefunc and it includes a crosstab function. On Stack Overflow I learn that you import this function. But from where and how?  The Postgres documentation is quite clear and intelligible to a retired English professor like me, but there is nothing in the Postgres documentation about how to do the import, and a search for ‘import modules’ yields nothing.

I tried to emulate a Stack overflow query that does what I want to do, but got an error message saying that the crosstab function doesn’t exist. I tried

CREATE EXTENSION IF NOT EXISTS tablefunc;

but it did nothing. It doesn’t seem to work  as import statements in Python do
-----
Martin Mueller
Professor emeritus of English and Classics
Northwestern University