pghero on PostgresApp pg_stat_statements must be loaded via shared_preload_libraries

You have to restart the Postgresql Server in order to load the shared library and afterwards execute

CREATE EXTENSION pg_stat_statements;

in the database you want to monitor.

Additionally you will have to edit the following line in postgresql.conf

shared_preload_libraries=""

To

shared_preload_libraries="pg_stat_statements"

A restart of the PSQL service is required.